feat(android): play the original file — decode Dolby/DTS audio with FFmpeg

Android ships no AC-3, E-AC-3, DTS or TrueHD decoders; they are licensed
codecs, present only where a vendor paid for them. The ROD2-W09 tablet has a
vendor DTS decoder and no AC-3/E-AC-3 at all. So every film with Dolby audio
was re-encoded by the server, for streaming and for download alike, and a
transcoded download has no Content-Length and ignores Range: ~1 MB/s,
restarting from byte zero on every network blip.

ExoPlayer now carries Jellyfin's media3 FFmpeg audio decoder in extension
mode ON (platform decoders first, FFmpeg for what they lack), and
CodecDetector reports its codecs so the device profile and the download
policy agree with what actually decodes. The download policy judges audio
against the renderer that will play the file (renderer_can_decode_audio)
instead of the webview's list, so Android downloads are always the direct
copy — a 910 MB E-AC-3 5.1 episode downloaded in 94 s and played offline.

The webview video path is removed on Android: it decodes none of these
codecs, so a stored "native video off" would play every original-file
download silent. Rust reports webview_video_fallback (false on Android, true
only beside mpv native video on Linux); Settings offers the switch and the
player honours it only then. Linux keeps the fallback and, with it, the
server transcode for undecodable audio.

The decoder is GPL-3.0; the distributed APK carries its terms and the source
stays MIT (THIRD_PARTY_NOTICES.md). The on-device remux spec this replaces is
folded into 05-platform-backends.md and deleted.

DR-293, UT-259, UT-262.

src/lib/stores/nativeVideo.ts

@@ -116,6 +116,23 @@ function createExperimentalNativeVideoStore() {
  */
 export const experimentalNativeVideo = createExperimentalNativeVideoStore();
 
+/**
+ * Whether video should take the native path, given the user's stored choice
+ * and whether this platform lets the user choose at all.
+ *
+ * On Android the answer is always native: ExoPlayer is the only video renderer
+ * there, and the webview element decodes none of the AC-3/E-AC-3/DTS/TrueHD
+ * that ExoPlayer plays through the FFmpeg extension — so a stored "off" would
+ * turn every original-file download into a silent film (DR-293). Rust reports
+ * whether a fallback exists (`webviewVideoFallback`); only then does the
+ * stored choice count.
+ *
+ * TRACES: UR-003, UR-071 | DR-293 | UT-262
+ */
+export function nativeVideoWanted(storedChoice: boolean, webviewVideoFallback: boolean): boolean {
+  return webviewVideoFallback ? storedChoice : true;
+}
+
 function createNativeVideoActiveStore() {
   const { subscribe, set } = writable<boolean>(false);
 

src/lib/stores/nativeVideo.wanted.test.ts

@@ -0,0 +1,17 @@
+import { describe, it, expect } from "vitest";
+import { nativeVideoWanted } from "./nativeVideo";
+
+// TRACES: UR-003, UR-071 | DR-293 | UT-262
+describe("nativeVideoWanted", () => {
+  it("ignores a stored 'off' where there is no webview fallback (Android)", () => {
+    // Someone who once switched native video off on Android must not be
+    // routed to the webview, which plays original-file downloads silent.
+    expect(nativeVideoWanted(false, false)).toBe(true);
+    expect(nativeVideoWanted(true, false)).toBe(true);
+  });
+
+  it("honours the stored choice where a fallback exists (Linux beside mpv)", () => {
+    expect(nativeVideoWanted(false, true)).toBe(false);
+    expect(nativeVideoWanted(true, true)).toBe(true);
+  });
+});