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/app/mod.rs

@@ -46,6 +46,7 @@ pub enum AppMessage {
     NavigateToSettings,
     NavigateBack,
     ToggleNotifications(bool),
+    ToggleEchoCancellation(bool),
     CustomSoundPathChanged(Sound, String),
 }
 
@@ -196,6 +197,7 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
                     ticket: state.ticket_input.clone(),
                     input_device,
                     output_device,
+                    echo_cancellation: state.config.echo_cancellation_enabled,
                 });
             }
         }
@@ -208,6 +210,7 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
                 ticket: "create".to_string(),
                 input_device,
                 output_device,
+                echo_cancellation: state.config.echo_cancellation_enabled,
             });
         }
         AppMessage::LeavePressed => {
@@ -331,6 +334,11 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
             state.config.save();
             notify::set_enabled(enabled);
         }
+        AppMessage::ToggleEchoCancellation(enabled) => {
+            state.config.echo_cancellation_enabled = enabled;
+            state.config.save();
+            // Applied on the next join, since the audio graph is rebuilt then.
+        }
         AppMessage::CustomSoundPathChanged(sound, path) => {
             let path_opt = if path.trim().is_empty() { None } else { Some(path) };
             match sound {
@@ -515,6 +523,11 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                         text(format!("Mic Sensitivity (Noise Gate): {:.1}%", state.config.noise_gate_threshold * 100.0)).size(14).color(color_subtext),
                         slider(0.0..=0.1, state.config.noise_gate_threshold, AppMessage::NoiseGateChanged).step(0.001),
                         text("Smoothly fades out audio below this level. 0% disables the gate.").size(11).color(color_subtext),
+                        vertical_space(4.0),
+                        checkbox(state.config.echo_cancellation_enabled)
+                            .label("Echo cancellation")
+                            .on_toggle(AppMessage::ToggleEchoCancellation),
+                        text("Cancels speaker echo + suppresses noise (PipeWire). Takes effect on your next room join.").size(11).color(color_subtext),
                     ].spacing(8).width(iced::Length::Fill),
                     column![
                         text("Network Privacy").size(14).color(color_subtext),