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/components/player/VideoPlayer.scrubRegression.test.ts

@@ -44,6 +44,18 @@ vi.mock("$lib/stores/nativeVideo", async (importOriginal) => {
   };
 });
 
+// The webview path only exists where Rust offers a fallback from the native
+// renderer — beside mpv native video on Linux; never on Android since DR-293,
+// where a stored "off" is ignored. These tests guard that path's scrubbing, so
+// they declare a platform that has it.
+vi.mock("$lib/services/playbackCapabilities", () => ({
+  getPlaybackCapabilities: async () => ({
+    usesWebviewAudio: false,
+    supportsNativeVideo: true,
+    webviewVideoFallback: true,
+  }),
+}));
+
 vi.mock("@tauri-apps/api/event", () => ({
   listen: vi.fn(async (channel: string, handler: any) => {
     channelHandlers[channel] = handler;

src/lib/components/player/VideoPlayer.svelte

@@ -41,7 +41,8 @@
     type Html5ElementBridge,
   } from "$lib/player/adapters";
   import { createRustReportHost } from "$lib/player/adapters/rustReportHost";
-  import { experimentalNativeVideo } from "$lib/stores/nativeVideo";
+  import { experimentalNativeVideo, nativeVideoWanted } from "$lib/stores/nativeVideo";
+  import { getPlaybackCapabilities } from "$lib/services/playbackCapabilities";
   import {
     enableNativeVideoCompositing,
     disableNativeVideoCompositing,
@@ -1020,7 +1021,12 @@
         // it is off we must also stop the native backend that player_play_item
         // just started, or ExoPlayer and the <video> element both decode the
         // same stream and the audio doubles.
-        if (!useHtml5Element && !$experimentalNativeVideo) {
+        // The stored choice only counts where Rust says a webview fallback
+        // exists — never on Android, where the webview would play an
+        // original-file download silent (DR-293).
+        const { webviewVideoFallback } = await getPlaybackCapabilities();
+        const wantNative = nativeVideoWanted($experimentalNativeVideo, webviewVideoFallback);
+        if (!useHtml5Element && !wantNative) {
           log.debug("Native backend available but experimentalNativeVideo is off - using HTML5");
           useHtml5Element = true;
           try {
@@ -1077,7 +1083,7 @@
             bridge: adapterBridge,
             // useHtml5Element is already the resolved decision above, so the
             // flag has had its say; pass it through for the invariant check.
-            experimentalNativeVideo: $experimentalNativeVideo,
+            experimentalNativeVideo: wantNative,
           });
           // No-op for the native adapter, which owns no DOM element.
           playerAdapter.attach(videoElement);

src/lib/components/player/VideoPlayer.touchScrub.test.ts

@@ -47,6 +47,18 @@ vi.mock("$lib/stores/nativeVideo", async (importOriginal) => {
   };
 });
 
+// The webview path only exists where Rust offers a fallback from the native
+// renderer — beside mpv native video on Linux; never on Android since DR-293,
+// where a stored "off" is ignored. These tests guard that path's scrubbing, so
+// they declare a platform that has it.
+vi.mock("$lib/services/playbackCapabilities", () => ({
+  getPlaybackCapabilities: async () => ({
+    usesWebviewAudio: false,
+    supportsNativeVideo: true,
+    webviewVideoFallback: true,
+  }),
+}));
+
 vi.mock("@tauri-apps/api/event", () => ({
   listen: vi.fn(async (channel: string, handler: any) => {
     channelHandlers[channel] = handler;