fix(downloads): media-library UI review fixes (E7)

Deep-review remediation on the frontend, all findings adversarially verified:

- LibraryPlayer picks audio vs video from the canonical media_kind (single source), not a
  hardcoded container whitelist, so an uploaded audio file with an uncommon/absent container
  renders in <audio> instead of a broken video box (legacy null media_kind keeps the heuristic).
- UploadDialog aborts the in-flight upload when the dialog is closed by X/ESC/backdrop/Back
  (not only the footer Cancel); drag highlight uses an enter/leave depth counter to stop the
  flicker; the progress bar gets a stable role=progressbar aria-label with the value on aria-valuenow.
- DownloadCenter's Shared tab renders honest loading/error states instead of a blank panel, and
  the library/shared row-action gating moves to a single tested helper (lib/downloadActions).

Tests: downloadActions gating matrix, the uploadReq XHR path (abort/401/422-localize/onerror),
and the media_kind-driven audio/video branch.

i18n: downloads.upload.uploadProgress (HU + EN).

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

@@ -1,9 +1,9 @@
 // @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.
+// element. video/audio is chosen from the stored media_kind (the single source), NOT a container
+// list — so an audio upload with an uncommon/absent container still renders <audio src=/file>. i18n
+// is key-passthrough here.
 import { describe, it, expect, afterEach, vi } from "vitest";
 import { render, cleanup } from "@testing-library/react";
 
@@ -89,11 +89,37 @@ describe("LibraryPlayer", () => {
     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={() => {}} />);
+  it("plays an audio-only asset (media_kind='audio') in an <audio> element", () => {
+    render(
+      <LibraryPlayer
+        job={withAsset({ media_kind: "audio", 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();
   });
+
+  it("uses media_kind — an audio upload with an uncommon container still renders <audio>", () => {
+    // The exact bug the fix targets: the old container whitelist (m4a/mp3/…) would drop an .m4b/.weba
+    // audio upload into a <video> box. media_kind='audio' is authoritative regardless of container.
+    render(
+      <LibraryPlayer
+        job={withAsset({ media_kind: "audio", container: "m4b" })}
+        onClose={() => {}}
+      />,
+    );
+    expect(document.body.querySelector("audio")).not.toBeNull();
+    expect(document.body.querySelector("video")).toBeNull();
+  });
+
+  it("falls back to the container heuristic for a legacy null media_kind", () => {
+    // A pre-discriminator asset (media_kind null) still resolves audio vs video from dims+container.
+    render(
+      <LibraryPlayer job={withAsset({ media_kind: null, container: "mp3" })} onClose={() => {}} />,
+    );
+    expect(document.body.querySelector("audio")).not.toBeNull();
+  });
 });