fix(player): let the background-audio toggle govern backgrounding again

Locking the screen kept a video's audio playing whether or not the
background-audio button was on. Reported as "audio only mode is always
active even if not selected".

The button (UR-040) was built for the WebView <video> path, where losing
visibility kills the decode: it chose between handing off to a native
audio stream and letting playback stop. Native video then became the
default renderer (DR-188), and on that path playback runs through
ExoPlayer inside a MediaSessionService -- a foreground media service
whose entire purpose is to keep playing while the app is hidden. Nothing
stopped it, and nothing in the codebase paused on background.

So the button governed a handoff that no longer had a gap to bridge.
There was no interruption to paper over, and a user who never touched it
got background playback anyway.

The gating made it self-concealing: MainActivity.onStop only dispatched
'jellytau-background' when backgroundAudioEnabled was already true. The
one notification that the app had gone away was itself conditional on the
setting, so with the button OFF nothing could react even in principle.
onStop and onStart now fire unconditionally and carry the two facts only
the activity knows -- whether the toggle is armed, and whether Android
put the window into picture-in-picture.

What to do about it is decided in Rust (player/background_policy.rs),
because it depends on whether the item has a picture to lose:

  video + toggle off  -> Pause
  video + toggle on   -> HandOffToAudio
  music, either       -> KeepPlaying   (no picture to give up)
  picture-in-picture  -> KeepPlaying   (the window is still on screen)

It takes no renderer parameter on purpose. Two renderers with two
behaviours and one toggle reaching only one of them is what produced the
defect; a rule that cannot see the renderer cannot reproduce it.

Two failure modes are deliberate. A decision call that fails leaves
playback alone rather than risking silence mid-listen. An event with no
detail -- older Kotlin against newer JS -- reads as "armed, not PiP",
degrading to the previous behaviour instead of pausing unexpectedly.

Foregrounding resumes only what backgrounding paused: a video the user
paused themselves before locking stays paused.

Written test-first per CLAUDE.md. The stub encoded today's behaviour
(nothing ever pauses) and failed exactly as reported --
`left: KeepPlaying, right: Pause` -- before the rule was implemented.

Verified on a device, R8-minified, both directions:

  [player_background_action] video=true armed=false pip=false -> Pause
  [player_background_action] video=true armed=true  pip=false -> HandOffToAudio

UR-040 / DR-224 / UT-211.

src/lib/components/player/VideoPlayer.svelte

@@ -4,7 +4,7 @@
   import { get } from "svelte/store";
   import { goto } from "$app/navigation";
   import { commands } from "$lib/api/bindings";
-  import type { JRayActor, StreamingQuality } from "$lib/api/bindings";
+  import type { JRayActor, StreamingQuality, BackgroundAction } from "$lib/api/bindings";
   import { listen } from "@tauri-apps/api/event";
   import Hls from "hls.js";
   import type { MediaItem } from "$lib/api/types";
@@ -63,6 +63,7 @@
   import {
     setBackgroundAudioEnabled,
     subscribeAppBackgrounded,
+    type BackgroundSignal,
     subscribeAppForegrounded,
   } from "$lib/utils/backgroundAudio";
   import { platform } from "@tauri-apps/plugin-os";
@@ -825,7 +826,7 @@
     // flip the component into HTML5 mode). Unsubscribers go into nativeUnlisteners
     // so onDestroy tears them down.
     if (backgroundAudioSupported) {
-      nativeUnlisteners.push(subscribeAppBackgrounded(enterBackgroundAudioHandoff));
+      nativeUnlisteners.push(subscribeAppBackgrounded(onAppBackgrounded));
       nativeUnlisteners.push(subscribeAppForegrounded(exitBackgroundAudioHandoff));
     }
 
@@ -1722,6 +1723,9 @@
   const backgroundAudioSupported = platform() === "android";
   let backgroundAudioOn = $state(false); // v1: default OFF each session
   let handoffState: BackgroundAudioState = { ...initialHandoffState };
+  // Set when backgrounding paused playback, so foregrounding resumes only
+  // what we stopped -- never something the user paused themselves.
+  let pausedByBackgrounding = false;
 
   function toggleBackgroundAudio() {
     backgroundAudioOn = !backgroundAudioOn;
@@ -1737,6 +1741,43 @@
 
   // App went to background/locked while background-audio is armed: hand off to
   // native audio and stop the WebView video decode.
+  async function onAppBackgrounded(signal: BackgroundSignal) {
+    // What to do is a domain decision, not a presentation one: it depends on
+    // whether the item has a picture to lose, which is Rust's to know. This used
+    // to be decided implicitly by Kotlin gating the event on the toggle, which
+    // is why the native path -- whose media service keeps playing regardless --
+    // ignored the toggle entirely (DR-224).
+    let action: BackgroundAction;
+    try {
+      action = await commands.playerBackgroundAction(
+        signal.backgroundAudioArmed,
+        signal.inPictureInPicture,
+      );
+    } catch (e) {
+      // Never leave playback in an undefined state because a decision call
+      // failed. Continuing is the old behaviour and the safer default: it
+      // cannot silently stop something the user is listening to.
+      log.warn("Background action lookup failed; leaving playback alone:", e);
+      return;
+    }
+
+    log.debug("Background action:", action);
+    switch (action) {
+      case "keepPlaying":
+        return;
+      case "pause":
+        // The user did not ask for background playback. Remember that WE paused
+        // it, so returning to the foreground can resume rather than leaving a
+        // video mysteriously stopped.
+        pausedByBackgrounding = isPlaying;
+        if (isPlaying) await playerController.pause();
+        return;
+      case "handOffToAudio":
+        await enterBackgroundAudioHandoff();
+        return;
+    }
+  }
+
   async function enterBackgroundAudioHandoff() {
     if (!shouldEnterBackgroundAudio(backgroundAudioOn, handoffState)) return;
     // `currentTime` is the component's authoritative ABSOLUTE position (the RAF
@@ -1797,6 +1838,20 @@
   // App returned to foreground: stop native audio, reload the WebView <video> at
   // the position native reached, and restore play/pause.
   async function exitBackgroundAudioHandoff() {
+    // Resume what backgrounding paused, before the handoff check: the pause path
+    // and the handoff path are mutually exclusive, and this one leaves no
+    // handoff state to unwind. Only resumes if WE paused it -- a user who
+    // paused before locking the screen stays paused.
+    if (pausedByBackgrounding) {
+      pausedByBackgrounding = false;
+      try {
+        await playerController.play();
+      } catch (e) {
+        log.warn("Failed to resume after backgrounding:", e);
+      }
+      return;
+    }
+
     if (!shouldExitBackgroundAudio(handoffState)) return;
     // Read the native player's state BEFORE exiting — the exit stops it. If the
     // user hit pause on the lockscreen while backgrounded, that pause must