feat: echo cancellation via PipeWire module-echo-cancel

Add an opt-in Echo Cancellation toggle (Settings) that routes the call
through PipeWire's module-echo-cancel (WebRTC AEC + noise suppression + AGC)
instead of running an in-process canceller. PipeWire already sees both the
mic and the speaker monitor, so it handles the echo-reference alignment for
free and we avoid a C++ DSP dependency.

src/audio/echo_cancel.rs (new):
- enable(real_source, real_sink) loads the module via pactl (aec_method=webrtc),
  bound to the chosen devices with source_master/sink_master (defaults if unset),
  waits for the virtual nodes to appear, and returns an RAII guard that unloads
  the module on drop. Best-effort pre-clean of a stale instance from a crashed run.
- EC_SOURCE / EC_SINK are the virtual cleaned-mic source and reference sink.

core: when echo_cancellation is set on Join, load the module and point capture
at EC_SOURCE / playback at EC_SINK; stash the guard in ActiveSession so it
unloads on shutdown (after the audio streams release the nodes). Any failure
logs + warns the UI and falls back to the direct devices — never blocks the call.

config: new echo_cancellation_enabled (serde default false). app: Settings
checkbox under Mic Sensitivity, applied on next room join. An ignored live
smoke test covers the real load/unload path.

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

src/config.rs

@@ -47,6 +47,10 @@ pub struct AppConfig {
     pub noise_gate_threshold: f32,
     #[serde(default)]
     pub network_mode: NetworkMode,
+    /// Route audio through PipeWire's echo-cancel module (AEC + noise suppression).
+    /// Takes effect on the next room join. Off by default.
+    #[serde(default)]
+    pub echo_cancellation_enabled: bool,
     #[serde(default = "default_true")]
     pub notifications_enabled: bool,
     #[serde(default)]
@@ -74,6 +78,7 @@ impl Default for AppConfig {
             output_device: "".to_string(),
             noise_gate_threshold: 0.01,
             network_mode: NetworkMode::default(),
+            echo_cancellation_enabled: false,
             notifications_enabled: true,
             custom_sound_self_join: None,
             custom_sound_peer_join: None,