fix(player): recover a failed stream on Linux instead of stopping (DR-130)

A recoverable player error meant "playback is over": the frontend's error
handler stopped the player unconditionally, so a wifi blip killed the
track. Android already decides in its JNI callback, but MpvBackend is
constructed before PlayerController exists, so its event thread has no
controller to ask.

So MPV reports the failure and the frontend echoes it into the new
player_recover_stream command — the same shape as PlaybackEnded ->
player_on_playback_ended, keeping the decision in Rust. The command
re-opens the stream where it stopped, with the existing attempt budget
and backoff, and returns whether it handled it; only a false answer
falls through to the old stop path.

Android now reports the errors it has already declined as
*unrecoverable*, so the echo never asks the same question twice.

TRACES: UR-004, UR-040 | DR-130 | UT-117

src-tauri/src/player/mpv_backend.rs

@@ -561,7 +561,7 @@ impl PlayerBackend for MpvBackend {
     /// so at EOF it stops resolving and a bare `unwrap_or(0.0)` reported 0:00 at
     /// exactly the moment end-of-file handling asks where playback reached.
     ///
-    /// TRACES: UR-005 | DR-130 | UT-118
+    /// TRACES: UR-005 | DR-130 | UT-121
     fn position(&self) -> f64 {
         let live = self.mpv.get_property::<f64>("time-pos").ok();
         self.observed.lock_safe().position_or_last(live)
@@ -570,7 +570,7 @@ impl PlayerBackend for MpvBackend {
     /// Total duration — live, or the last one observed. Unloaded at EOF for the
     /// same reason as `position`.
     ///
-    /// TRACES: UR-005 | DR-130 | UT-118
+    /// TRACES: UR-005 | DR-130 | UT-121
     fn duration(&self) -> Option<f64> {
         let live = self.mpv.get_property::<f64>("duration").ok();
         self.observed.lock_safe().duration_or_last(live)