fix(player): make lockscreen transport reach background audio (DR-097)

Pausing from the lockscreen did nothing while a video's audio played in
the background. The handoff starts native ExoPlayer audio and only then
tears the WebView <video> down, and that teardown fires a DOM `pause`
the frontend reports like any other — leaving html5_playing = Some(false).
Transport therefore stayed aimed at the element: the lockscreen pause
emitted a ControlCommand into a <video> that no longer existed while the
native player carried on.

The controller now tracks a background-audio handoff explicitly. Entering
one hands transport authority to the native backend and drops the dying
element's state/position/media-loaded reports, which also stop flipping
the UI to paused and dragging the position backwards. Exiting restores
the element as the player.

A lockscreen pause also has to survive the return to the foreground: the
video used to resume from a snapshot taken at handoff time, undoing the
pause on the way back in. shouldResumeOnForeground() lets an explicit
`paused` from the player override that snapshot.

TRACES: UR-040, UR-005 | DR-052, DR-097

src/lib/components/player/backgroundAudioHandoff.ts

@@ -55,3 +55,22 @@ export function shouldEnterBackgroundAudio(
 export function shouldExitBackgroundAudio(state: BackgroundAudioState): boolean {
   return state.active;
 }
+
+/**
+ * Whether the `<video>` should start playing again once it reloads on foreground.
+ *
+ * `wasPlaying` is what the video was doing when we handed off, but the native
+ * audio player kept going after that — and the lockscreen/notification can pause
+ * it while backgrounded. The player is the authoritative source of play/pause,
+ * so an explicit `paused` from it overrides the handoff snapshot; anything less
+ * definite (loading, seeking, already-stopped, no state at all) falls back to
+ * the snapshot.
+ *
+ * TRACES: UR-040, UR-005 | DR-052 | UT-060
+ */
+export function shouldResumeOnForeground(
+  wasPlaying: boolean,
+  nativeStateKind: string | undefined
+): boolean {
+  return wasPlaying && nativeStateKind !== "paused";
+}