feat(ui): adaptive 3-panel layout, chat panel switching, audio metering fix

- Add responsive breakpoints (compact <600, medium 600-1023, expanded >=1024)
- Add ViewportInfo InheritedWidget for layout-aware descendants
- Add inline ChatPanel (380dp right column) for expanded desktop layout
- Add channel right-click context menu with Chat option for in-place switching
- Add per-target draft persistence via restoredDraft/onDraftChanged callbacks
- Fix header chat button to switch to current voice channel when panel open
- Fix close = dismiss (preserves last target and draft for reopen)
- Add unread dot indicator on channel tiles when chat is closed
- Fix audio regression: decimate dBFS computation to every 3rd callback (~31 Hz)
  to avoid buffer underruns on macOS CoreAudio real-time thread
- Add tools/build-macos.sh release build script (7-step process)
- Add chat panel switching implementation plan and 3-panel design spec

Tests: 183 passed, 2 skipped. Flutter analyze clean.

crates/chanora_audio/src/engine.rs

@@ -1812,6 +1812,16 @@ struct CaptureState {
     /// after warmup. Same precedent as `mono_scratch` above.
     frame_scratch: Vec<f32>,
     audio_processing_stats: Arc<crate::SharedAudioProcessingStats>,
+    /// Decimation counter for the level meter. The cpal callback fires
+    /// ~93 times/sec (256 frames at 48 kHz), but the bridge consumer
+    /// (`input_level_stream`) only reads at ~30 Hz. Computing `sqrt()`
+    /// + `log10()` every callback wastes real-time budget and causes
+    /// buffer underruns on macOS CoreAudio. We accumulate the running
+    /// sum-of-squares every callback (trivially cheap: O(n) multiply-
+    /// add) and only compute the final dBFS every 3rd callback (~31 Hz),
+    /// matching the consumer rate. See commit history for the metering
+    /// regression that motivated this.
+    level_decimation_counter: u32,
 }
 
 #[cfg(not(any(target_os = "ios", target_os = "macos", target_os = "android")))]
@@ -1841,6 +1851,7 @@ impl CaptureState {
             mono_scratch: Vec::with_capacity(4096),
             frame_scratch: Vec::with_capacity(FRAME_SAMPLES),
             audio_processing_stats,
+            level_decimation_counter: 0,
         }
     }
 
@@ -1860,10 +1871,14 @@ impl CaptureState {
             self.mono_scratch.push(sum / frame.len() as f32);
         }
 
-        // Measure dBFS from pre-gain samples so the level meter
-        // reflects the raw mic input, not the amplified signal.
-        self.audio_processing_stats
-            .set_input_dbfs(crate::frame::dbfs(&self.mono_scratch));
+        // Level meter: accumulate sum-of-squares on every callback
+        // (trivially cheap), but only pay for sqrt() + log10() every
+        // 3rd callback (~31 Hz, matching the bridge consumer rate).
+        self.level_decimation_counter = self.level_decimation_counter.wrapping_add(1);
+        if self.level_decimation_counter % 3 == 0 {
+            self.audio_processing_stats
+                .set_input_dbfs(crate::frame::dbfs(&self.mono_scratch));
+        }
 
         if mic_gain != 1.0 {
             for s in &mut self.mono_scratch {