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.

apps/chanora_flutter/lib/widgets/connect_widgets.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
+import '../design/breakpoints.dart';
 import '../l10n/generated/app_localizations.dart';
 import '../src/rust/api.dart' as rust;
 
@@ -114,7 +115,6 @@ class _ConnectFormState extends State<ConnectForm> {
         const SizedBox(height: 16),
         LayoutBuilder(
           builder: (context, constraints) {
-            const stackedActionsMaxWidth = 400.0;
             final connectButton = FilledButton.icon(
               icon: const Icon(Icons.login),
               label: Text(l10n.connectAction),
@@ -125,7 +125,8 @@ class _ConnectFormState extends State<ConnectForm> {
               label: Text(l10n.bookmarkAddAction),
               onPressed: widget.onAddBookmark,
             );
-            if (constraints.maxWidth <= stackedActionsMaxWidth) {
+            if (constraints.maxWidth <=
+                ChanoraBreakpoints.connectActionsStackMaxWidth) {
               return Column(
                 crossAxisAlignment: CrossAxisAlignment.stretch,
                 children: [