feat: MVP of MediaSessionCompat Android 13+ integration

docs/TODO.md

@@ -34,16 +34,31 @@ A dedicated settings screen where the user can view every setting and reset it.
   `PlayerViewModel`/`AppScreen`).
 - Room to grow: password field, connection timeout, theme, keep-screen-on.
 
-## [ ] 3. Cast-style volume control
+## [x] 3. Cast-style volume + OS media integration
 
-When the MPD server is playing, present the volume control as a **"casting"
-style** remote-volume control — the way MALP (and Google Cast) do — making it
-visually clear you're controlling the *server's* output, not the phone's.
+Done — full MALP-style **OS integration**, not just in-app. Architecture:
 
-- Functionally we already send `setvol` to the server (`MpdClient.setVolume`);
-  this is mostly a UX/affordance change: a cast icon, "Casting to <host>" label,
-  distinct styling for remote vs local volume.
-- Reference behaviour: MALP.
+- **`MpdConnectionManager`** (app-scoped, held by `MusicRemoteApplication`) now
+  owns the single `MpdClient`, so the connection survives Activity recreation and
+  runs while the service is up. `PlayerViewModel` is a thin delegate over it.
+- **`PlaybackService`** — a `mediaPlayback` foreground service hosting a
+  `MediaSessionCompat`:
+  - **Cast-style volume**: `setPlaybackToRemote(VolumeProviderCompat)` (absolute,
+    0–100). The OS routes hardware volume keys to the *server* volume
+    **system-wide, even when the app is backgrounded** (verified: 50→40 via
+    injected keys from the launcher), and shows the remote-volume UI. Replaced
+    the earlier in-app `onKeyDown` hack. Rapid presses accumulate via
+    `pendingVolume`.
+  - **Now-playing notification / QS / lock-screen** via a `MediaStyle`
+    notification bound to the session, with prev/play-pause/next actions
+    (`MediaButtonReceiver`) and metadata/position mirrored from the flows.
+  - Needs `POST_NOTIFICATIONS` (requested in `MainActivity`) +
+    `FOREGROUND_SERVICE[_MEDIA_PLAYBACK]`.
+- In-app **`CastIndicator`** ("Controlling <host>") still shown above the slider.
+
+Notes: verified on the FiiO via injected key events (a rotary volume knob may not
+emit `VOLUME_UP/DOWN` — hardware-dependent). Album art in the notification/session
+is pending item #5.
 
 ## [ ] 4. Library browse — albums
 
@@ -85,3 +100,8 @@ and falls back to the connect screen. MALP does not do this.
   **auto-reconnect transparently** (re-open connections, re-issue `idle`, resync
   state) and keep showing the player. Consider a keepalive ping and, for
   backgrounded playback control, a foreground service / partial wakelock.
+- PARTIALLY ADDRESSED by item #3: the `mediaPlayback` foreground service now
+  keeps the process/connection alive in the background, which should stop Doze /
+  wifi power-save from tearing the sockets down (the most likely cause). Still
+  worth verifying over a long idle, and adding transparent auto-reconnect +
+  keepalive ping as defense-in-depth (e.g. against MPD `connection_timeout`).