feat: local call recording (your mic + incoming mix) to WAV

Opt-in recording of the full call as you experienced it. New dep-free
src/audio/recorder.rs: a canonical mono S16LE WavWriter (header patched on
finalize) plus a Recorder that buffers your transmitted mic in a bounded FIFO
and sums it, sample-aligned, with each incoming-mix frame the playout mixer
produces. The two independently-clocked streams stay aligned via the FIFO
(capped at ~200ms so drift lag can't grow without bound); silent stretches
record the incoming mix alone. Dep-free UTC timestamp -> sortable filename.

Wiring: CoreCommand::SetRecording toggles an Arc<Mutex<Option<Recorder>>> gated
by an is_recording flag (so the capture/mixer hot paths only lock while actually
recording); capture pushes post-gate mic, the mixer writes the pre-deafen mix.
Recording finalizes on stop, room leave, and room switch. UI: a Record/Stop
button in the controls and a red "● REC m:ss" pill in the room header;
core-confirmed Recording{Started,Stopped} events drive the UI flag so a failed
start can't lie. Files land in ~/peerspeak-recordings/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/messages.rs

@@ -26,6 +26,9 @@ pub enum CoreCommand {
     /// Set the relay/discovery posture. Takes effect on the next room join,
     /// since the endpoint is (re)built then.
     SetNetworkMode(NetworkMode),
+    /// Start/stop recording the call to a local WAV (your mic + the incoming
+    /// mix). No-op start if already recording / not in a call.
+    SetRecording(bool),
 }
 
 #[derive(Debug, Clone)]
@@ -44,5 +47,9 @@ pub enum UiEvent {
     /// Raw (pre-gate, pre-mute) normalized RMS of the local mic, `0.0..=1.0`,
     /// for the settings level meter. Throttled to ~10/sec.
     MicLevel(f32),
+    /// Call recording started; carries the absolute WAV path being written.
+    RecordingStarted { path: String },
+    /// Call recording stopped; carries the finished WAV path.
+    RecordingStopped { path: String },
     Error(String),
 }