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.

docs/architecture/05-platform-backends.md

@@ -95,8 +95,9 @@ flowchart LR
 **Location**: `src/lib/player/html5Adapter.ts`, `src/lib/player/index.ts`, report commands in
 `src-tauri/src/commands/player/timers.rs`
 
-Video on desktop (Linux WebKitGTK) — and, per current interim behavior, Android — is rendered by an
-HTML5 `<video>`/HLS element **inside the webview**. libmpv is initialized audio-only (`vo=null`,
+Video on desktop (Linux WebKitGTK) is rendered by an HTML5 `<video>`/HLS element **inside the
+webview**. Android no longer uses this path for video — see *The webview is not a video renderer on
+Android* below. libmpv is initialized audio-only (`vo=null`,
 `video=false`), so the native backend cannot render or observe this element. The `<video>` is therefore
 the real player, living outside Rust's reach.
 
@@ -287,6 +288,50 @@ and the trait default is still a silent `Ok(())` rather than an error, so a back
 that omits the method still reports success. Flipping that default waits on the
 device verification.
 
+### Licensed audio codecs: the FFmpeg extension
+
+**TRACES**: UR-004, UR-071 | DR-293
+
+Android does not ship AC-3, E-AC-3, DTS or TrueHD decoders — they are licensed
+codecs, present only where a vendor paid for them. The ROD2-W09 test tablet has a
+vendor DTS decoder and no AC-3/E-AC-3 at all. ExoPlayer has no decoders of its
+own, so on such a device those tracks are undecodable, and before this every film
+with Dolby audio was re-encoded by the server — for streaming *and* for download.
+
+`JellyTauPlayer` builds ExoPlayer with `DefaultRenderersFactory` in
+`EXTENSION_RENDERER_MODE_ON`: the platform's decoders are tried first (a vendor DTS
+decoder stays in charge where there is one) and the FFmpeg audio renderer takes
+what they cannot decode. `CodecDetector` reports the extension's codecs beside the
+`MediaCodecList` ones, asking `FfmpegLibrary.supportsFormat` per MIME type rather
+than assuming, so a build whose native library failed to load reports only what
+the platform decodes. Rust's device profile and download policy read that list,
+which is what keeps "what we tell the server" and "what actually decodes" in step.
+
+The decoder is `org.jellyfin.media3:media3-ffmpeg-decoder` — Jellyfin's build of
+media3's FFmpeg extension, versioned `<media3 version>+N`. **Bump it in the same
+commit as media3.** It is GPL-3.0: the distributed APK carries those terms, the
+source stays MIT (see `THIRD_PARTY_NOTICES.md`). Its JNI methods are covered by the
+AAR's own consumer rules and by `-keep class androidx.media3.** { *; }` in
+`proguard-jellytau.pro`, which also keeps the renderer ExoPlayer loads reflectively.
+
+**Rejected:** re-encoding a download's audio on the device after it lands (a
+remux). It costs minutes of CPU and twice the disk per film, needs a pipeline
+state of its own, and does nothing for streaming. Decoding at playback fixes both
+paths with no extra step.
+
+### The webview is not a video renderer on Android
+
+ExoPlayer is Android's only video renderer. The HTML5 path used to be reachable
+through the `experimentalNativeVideo` setting (a *suppressor* of Rust's native
+choice), but the webview decodes none of the codecs above — so with the original
+file now downloaded as-is (DR-293), turning native video off would play every such
+download as a silent film. Rust reports `webview_video_fallback` in
+`PlaybackCapabilities`: **false on Android**, true only beside mpv native video on
+Linux, where the webview is still the tested fallback. The frontend offers the
+switch and honours a stored "off" only when it is true (`nativeVideoWanted` in
+`stores/nativeVideo.ts`), so a user who once switched it off on Android is not
+stranded on the silent path.
+
 ### The equalizer, and where its vocabulary lives
 
 **TRACES**: UR-027 | DR-030, IR-020