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/UploadDialog.tsx

@@ -1,4 +1,4 @@
-import { useRef, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useQueryClient } from "@tanstack/react-query";
 import { UploadCloud, File as FileIcon } from "lucide-react";
@@ -24,8 +24,16 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
   const [pct, setPct] = useState<number | null>(null); // null = idle; 0–100 while uploading
   const inputRef = useRef<HTMLInputElement>(null);
   const abortRef = useRef<AbortController | null>(null);
+  // dragenter/dragleave fire as the cursor crosses the drop zone's OWN children (icon/text), so a
+  // plain boolean flickers. Count enter vs leave and only clear the highlight at depth 0.
+  const dragDepth = useRef(0);
   const busy = pct !== null;
 
+  // Closing the dialog mid-upload (X / ESC / backdrop / browser Back all unmount us) must abort the
+  // in-flight XHR — otherwise it keeps streaming in the background and still toasts success. Only the
+  // footer Cancel wired the abort; this covers every other dismissal.
+  useEffect(() => () => abortRef.current?.abort(), []);
+
   const pick = () => inputRef.current?.click();
 
   const submit = async () => {
@@ -62,13 +70,19 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
       <button
         type="button"
         onClick={pick}
-        onDragOver={(e) => {
+        onDragEnter={(e) => {
           e.preventDefault();
+          dragDepth.current += 1;
           setDragOver(true);
         }}
-        onDragLeave={() => setDragOver(false)}
+        onDragOver={(e) => e.preventDefault()}
+        onDragLeave={() => {
+          dragDepth.current = Math.max(0, dragDepth.current - 1);
+          if (dragDepth.current === 0) setDragOver(false);
+        }}
         onDrop={(e) => {
           e.preventDefault();
+          dragDepth.current = 0;
           setDragOver(false);
           const f = e.dataTransfer.files?.[0];
           if (f) {
@@ -123,7 +137,16 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
 
       {busy && (
         <div className="mb-5" aria-live="polite">
-          <div className="h-2 rounded-full bg-surface overflow-hidden">
+          <div
+            className="h-2 rounded-full bg-surface overflow-hidden"
+            role="progressbar"
+            aria-valuenow={pct ?? 0}
+            aria-valuemin={0}
+            aria-valuemax={100}
+            // A STABLE descriptor as the name; the live percentage rides aria-valuenow (and the visible
+            // <p> below re-announces it via the aria-live wrapper) — don't bake the value into the name.
+            aria-label={t("downloads.upload.uploadProgress")}
+          >
             <div
               className="h-full bg-accent transition-[width] duration-150"
               style={{ width: `${pct ?? 0}%` }}