feat(downloads): in-app player for library items (E7 S4b)

Play a completed library item in-app: a Play button on Library rows (and
Shared rows with download access) opens a modal that streams the local file
into a native <video controls> (or <audio> for audio-only assets).

Rather than the YouTube IFrame PlayerModal or the Plex HLS PlexPlayer — both
bound to their own domain APIs and unable to point at a local file — this
reuses the shared Modal a11y shell (focus-trap, ESC, layer-stack, portal) and
the range-aware /api/downloads/{id}/file endpoint. S4a's universal-codec
guarantee (mp4/H.264/AAC + m4a/mp3) makes native playback always work, so seek,
volume, fullscreen and speed come for free with zero transcode.

frontend/src/components/DownloadCenter.tsx

@@ -9,6 +9,7 @@ import {
   Link2,
   Pause,
   Play,
+  PlayCircle,
   Plus,
   RotateCcw,
   Scissors,
@@ -26,6 +27,7 @@ const DownloadDialog = lazy(() => import("./DownloadDialog"));
 const ProfileEditor = lazy(() => import("./ProfileEditor"));
 const VideoEditor = lazy(() => import("./VideoEditor"));
 const ShareDialog = lazy(() => import("./ShareDialog"));
+const LibraryPlayer = lazy(() => import("./LibraryPlayer"));
 import { useConfirm } from "./ConfirmProvider";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { useMe } from "../lib/useMe";
@@ -50,6 +52,7 @@ export default function DownloadCenter() {
   const [manage, setManage] = useState(false);
   const [metaJob, setMetaJob] = useState<DownloadJob | null>(null);
   const [shareJob, setShareJob] = useState<DownloadJob | null>(null);
+  const [playJob, setPlayJob] = useState<DownloadJob | null>(null);
   const [editJob, setEditJob] = useState<DownloadJob | null>(null);
   const [errorJob, setErrorJob] = useState<DownloadJob | null>(null);
 
@@ -265,6 +268,9 @@ export default function DownloadCenter() {
             <div className="space-y-2">
               {library.map((job) => (
                 <DownloadRow key={job.id} job={job}>
+                  <IconBtn onClick={() => setPlayJob(job)} title={t("downloads.actions.play")}>
+                    <PlayCircle className="w-4 h-4" />
+                  </IconBtn>
                   {saveBtn(job)}
                   {job.can_download && (job.asset?.duration_s ?? 0) > 0 && (
                     <IconBtn onClick={() => setEditJob(job)} title={t("downloads.actions.edit")}>
@@ -333,6 +339,11 @@ export default function DownloadCenter() {
           <div className="space-y-2">
             {(sharedQ.data ?? []).map((job) => (
               <DownloadRow key={job.id} job={job}>
+                {job.can_download && (
+                  <IconBtn onClick={() => setPlayJob(job)} title={t("downloads.actions.play")}>
+                    <PlayCircle className="w-4 h-4" />
+                  </IconBtn>
+                )}
                 {job.can_download && saveBtn(job)}
                 {job.can_download && (job.asset?.duration_s ?? 0) > 0 && (
                   <IconBtn onClick={() => setEditJob(job)} title={t("downloads.actions.edit")}>
@@ -377,6 +388,7 @@ export default function DownloadCenter() {
           {manage && <ProfileEditor onClose={() => setManage(false)} />}
           {metaJob && <MetaEditModal job={metaJob} onClose={() => setMetaJob(null)} />}
           {shareJob && <ShareDialog job={shareJob} onClose={() => setShareJob(null)} />}
+          {playJob && <LibraryPlayer job={playJob} onClose={() => setPlayJob(null)} />}
           {editJob && <VideoEditor job={editJob} onClose={() => setEditJob(null)} />}
           {errorJob && (
             <ErrorModal

frontend/src/components/LibraryPlayer.dom.test.tsx

@@ -0,0 +1,98 @@
+// @vitest-environment jsdom
+//
+// E7·S4b — the in-app library player streams the completed item's LOCAL file into a native media
+// element (no transcode; S4a guarantees a browser-native container). A video asset (has pixel dims)
+// renders <video src=/file>; an audio-only asset (m4a/mp3, no dims) renders <audio src=/file>. i18n is
+// key-passthrough here.
+import { describe, it, expect, afterEach, vi } from "vitest";
+import { render, cleanup } from "@testing-library/react";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+});
+
+import LibraryPlayer from "./LibraryPlayer";
+import type { DownloadJob } from "../lib/api";
+
+const baseJob: DownloadJob = {
+  id: 7,
+  status: "done",
+  progress: 100,
+  speed_bps: null,
+  eta_s: null,
+  phase: null,
+  error: null,
+  error_code: null,
+  queue_pos: 0,
+  created_at: null,
+  source_kind: "youtube",
+  source_ref: "abc",
+  source_url: null,
+  poster_url: null,
+  profile_id: null,
+  spec: {
+    mode: "av",
+    max_height: null,
+    container: null,
+    audio_format: null,
+    vcodec: null,
+    embed_subs: false,
+    embed_chapters: false,
+    embed_thumbnail: false,
+    sponsorblock: false,
+  },
+  display_name: "Clip",
+  display_uploader: null,
+  display_uploader_url: null,
+  extra_links: [],
+  can_download: true,
+  expired: false,
+  asset: null,
+};
+
+const withAsset = (over: Partial<NonNullable<DownloadJob["asset"]>>): DownloadJob => ({
+  ...baseJob,
+  asset: {
+    id: 10,
+    status: "ready",
+    title: "Clip",
+    uploader: null,
+    uploader_url: null,
+    upload_date: null,
+    duration_s: 12,
+    size_bytes: 1000,
+    width: null,
+    height: null,
+    container: null,
+    thumbnail_url: null,
+    expires_at: null,
+    ...over,
+  },
+});
+
+afterEach(cleanup);
+
+describe("LibraryPlayer", () => {
+  it("plays a video asset in a <video> pointed at the range-aware /file endpoint", () => {
+    render(
+      <LibraryPlayer
+        job={withAsset({ width: 1280, height: 720, container: "mp4" })}
+        onClose={() => {}}
+      />,
+    );
+    const video = document.body.querySelector("video");
+    expect(video).not.toBeNull();
+    expect(video!.getAttribute("src")).toBe("/api/downloads/7/file");
+    // No <audio> element for a video asset.
+    expect(document.body.querySelector("audio")).toBeNull();
+  });
+
+  it("plays an audio-only asset (no dims, m4a) in an <audio> element", () => {
+    render(<LibraryPlayer job={withAsset({ container: "m4a" })} onClose={() => {}} />);
+    const audio = document.body.querySelector("audio");
+    expect(audio).not.toBeNull();
+    expect(audio!.getAttribute("src")).toBe("/api/downloads/7/file");
+    expect(document.body.querySelector("video")).toBeNull();
+  });
+});

frontend/src/components/LibraryPlayer.tsx

@@ -0,0 +1,57 @@
+import { useMemo } from "react";
+import { useTranslation } from "react-i18next";
+import Modal from "./Modal";
+import { api, type DownloadJob } from "../lib/api";
+import { jobTitle } from "./downloadRow";
+
+// E7 S4b — in-app player for a completed library item. We do NOT reuse PlayerModal (a cross-origin
+// YouTube IFrame shell) or PlexPlayer (a Plex HLS/session shell): both are bound to their own domain
+// APIs and can't point at a local file. Instead we reuse the shared a11y shell (`Modal`: focus-trap,
+// ESC/back-to-close, scroll-lock, layer-stack, portal) and stream the local file into a NATIVE
+// <video>/<audio controls> — the same element VideoEditor already plays this URL in. S4a guarantees
+// every download is a browser-native container (mp4/H.264/AAC + m4a/mp3), so native playback (seek via
+// the range-aware /file endpoint, volume, fullscreen, speed) always works with zero transcode.
+export default function LibraryPlayer({ job, onClose }: { job: DownloadJob; onClose: () => void }) {
+  const { t } = useTranslation();
+  const fileUrl = useMemo(() => api.downloadFileUrl(job.id), [job.id]);
+  // Video vs audio-only: a video asset carries real pixel dimensions; audio (m4a/mp3) has none. Fall
+  // back to the container kind so a dimension-less-but-video asset still gets the <video> element.
+  const asset = job.asset;
+  const isAudio =
+    !(asset?.width && asset?.height) &&
+    !!asset?.container &&
+    ["m4a", "mp3", "opus", "ogg", "wav", "flac", "aac"].includes(asset.container.toLowerCase());
+  // Our self-hosted poster (always same-origin, never expires) is the primary; the source thumbnail is
+  // the fallback. Same precedence as the library-row thumbnail.
+  const poster = job.poster_url ? api.posterImageUrl(job.id) : (asset?.thumbnail_url ?? undefined);
+
+  return (
+    <Modal title={jobTitle(job)} onClose={onClose} maxWidth="max-w-4xl">
+      {isAudio ? (
+        <div className="flex flex-col items-center gap-4 py-2">
+          {poster ? (
+            <img
+              src={poster}
+              alt=""
+              className="w-40 h-40 rounded-xl object-cover border border-border"
+            />
+          ) : null}
+          <audio src={fileUrl} controls autoPlay className="w-full">
+            {t("downloads.player.unsupported")}
+          </audio>
+        </div>
+      ) : (
+        <video
+          src={fileUrl}
+          poster={poster}
+          controls
+          autoPlay
+          playsInline
+          className="w-full max-h-[70vh] rounded-lg bg-black object-contain"
+        >
+          {t("downloads.player.unsupported")}
+        </video>
+      )}
+    </Modal>
+  );
+}

frontend/src/i18n/locales/en/downloads.json

@@ -56,6 +56,7 @@
     "delete": "Remove",
     "retry": "Retry",
     "download": "Save to my device",
+    "play": "Play",
     "rename": "Rename",
     "editDetails": "Edit details",
     "share": "Share (copy link)",
@@ -64,6 +65,9 @@
     "redownload": "Re-download (rebuild file)",
     "removeShared": "Remove from my list"
   },
+  "player": {
+    "unsupported": "Your browser can't play this file."
+  },
   "empty": {
     "queue": "Nothing in the queue.",
     "done": "No downloads yet.",

frontend/src/i18n/locales/hu/downloads.json

@@ -56,6 +56,7 @@
     "delete": "Eltávolítás",
     "retry": "Újra",
     "download": "Mentés a gépemre",
+    "play": "Lejátszás",
     "rename": "Átnevezés",
     "editDetails": "Adatok szerkesztése",
     "share": "Megosztás (link másolása)",
@@ -64,6 +65,9 @@
     "redownload": "Újraletöltés (fájl újraépítése)",
     "removeShared": "Eltávolítás a listámból"
   },
+  "player": {
+    "unsupported": "A böngésződ nem tudja lejátszani ezt a fájlt."
+  },
   "empty": {
     "queue": "A sor üres.",
     "done": "Még nincs letöltés.",