feat(ui): in-call echo-cancellation toggle, synced with Settings

Add an "Echo cancellation" checkbox to the in-call control column (beside
Push-to-Talk). It binds to the same config.echo_cancellation_enabled flag and
ToggleEchoCancellation message as the Settings checkbox, so the two stay in
sync automatically (single source of truth; iced re-renders from it). A tooltip
is explicit that it applies on the NEXT room join — the current PipeWire-module
AEC is wired at join time and isn't hot-swappable mid-call. (A true live in-call
toggle falls out for free once the in-process EchoCanceller lands — AEC Stage E.)

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

src/app/mod.rs

@@ -1775,6 +1775,27 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                 column![]
             },
             vertical_space(20.0),
+            // Echo cancellation — same flag + message as the Settings checkbox, so
+            // toggling here and there stay in sync automatically (single source of
+            // truth: config.echo_cancellation_enabled). Tooltip is explicit that it
+            // applies on the NEXT join (the PipeWire-module AEC is wired at join
+            // time, not hot-swappable mid-call).
+            tooltip(
+                checkbox(state.config.echo_cancellation_enabled)
+                    .label("Echo cancellation")
+                    .on_toggle(AppMessage::ToggleEchoCancellation),
+                container(
+                    text("Cancels speaker echo + suppresses noise. Applies on your next room join.")
+                        .size(11)
+                        .color(color_text),
+                )
+                .padding(8)
+                .max_width(260.0)
+                .style(c_style(color_crust, color_surface, 6.0)),
+                iced::widget::tooltip::Position::Top,
+            )
+            .gap(8),
+            vertical_space(20.0),
             {
                 let (rec_kind, rec_label, rec_bg, rec_hover, rec_fg) = if state.recording {
                     (IconKind::Stop, "Stop Recording", color_red, color_maroon, color_crust)