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/design/breakpoints.dart

@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: Apache-2.0
+// Canonical layout breakpoints for Chanora.
+//
+// Aligned with Material 3 adaptive layout guidance:
+//   compact  < 600dp  — phone, narrow tablet
+//   medium   600–1023 — tablet portrait, small desktop window
+//   expanded ≥ 1024dp — desktop, tablet landscape
+//
+// 1024dp was chosen as the expanded threshold based on production app
+// research: Discord (member list at 1024px), Mattermost (RHS docked at
+// ≥ 1024px), and Rocket.Chat (contextual bar persistent at lg/1024px).
+
+/// Canonical breakpoint thresholds in logical pixels.
+///
+/// Use these instead of hardcoded pixel values in layout decisions.
+/// Migrate existing `_wideBreakpoint` / `_chatMobileBreakpoint` references
+/// to these named constants.
+class ChanoraBreakpoints {
+  ChanoraBreakpoints._();
+
+  /// Width at which the layout switches from compact to medium.
+  /// Below this: single-column mobile layout.
+  /// At/above: two-panel side-by-side layout.
+  static const double medium = 600;
+
+  /// Width at which the layout switches from medium to expanded.
+  /// Below this: chat opens as a pushed route.
+  /// At/above: three-panel layout with inline chat panel.
+  static const double expanded = 1024;
+
+  // Panel sizing constants.
+
+  /// Fixed width of the left voice/control panel.
+  static const double voicePanelWidth = 320;
+
+  /// Fixed width of the right chat panel (expanded layout only).
+  static const double chatPanelWidth = 380;
+
+  /// Horizontal gap between panels.
+  static const double panelGap = 12;
+
+  /// Desktop snackbar width cap (used when width ≥ [medium]).
+  static const double snackBarDesktopCap = 560;
+
+  /// Connect form action buttons switch from row to column below this width.
+  static const double connectActionsStackMaxWidth = 400;
+
+  /// Modal bottom sheet max height as fraction of screen height.
+  static const double modalSheetHeightFraction = 0.72;
+}
+
+/// Semantic layout class derived from viewport width.
+enum LayoutClass {
+  /// < 600dp — single-column mobile layout.
+  compact,
+
+  /// 600–1023dp — two-panel side-by-side layout.
+  medium,
+
+  /// ≥ 1024dp — three-panel layout with inline chat.
+  expanded,
+}
+
+/// Computes the current [LayoutClass] from viewport [width].
+LayoutClass layoutClassFromWidth(double width) {
+  if (width >= ChanoraBreakpoints.expanded) return LayoutClass.expanded;
+  if (width >= ChanoraBreakpoints.medium) return LayoutClass.medium;
+  return LayoutClass.compact;
+}

apps/chanora_flutter/lib/design/viewport_info.dart

@@ -0,0 +1,64 @@
+// SPDX-License-Identifier: Apache-2.0
+// Viewport info inherited widget for Chanora.
+//
+// Computes [LayoutClass] once per frame from the current [MediaQuery] size
+// and provides it to the entire widget subtree. Downstream widgets read
+// `ViewportInfo.of(context)` instead of calling `LayoutBuilder` or
+// `MediaQuery.sizeOf` directly for layout-class decisions.
+
+import 'package:flutter/widgets.dart';
+
+import 'breakpoints.dart';
+
+/// Inherited widget that exposes the current layout class and viewport
+/// dimensions to the entire subtree.
+///
+/// Insert this once near the top of the widget tree (inside the Scaffold
+/// body or equivalent). All descendants can then read
+/// `ViewportInfo.of(context)` to determine their layout behaviour.
+class ViewportInfo extends InheritedWidget {
+  /// Creates a [ViewportInfo].
+  const ViewportInfo({
+    super.key,
+    required this.layoutClass,
+    required this.width,
+    required this.height,
+    required super.child,
+  });
+
+  /// Current layout class derived from viewport width.
+  final LayoutClass layoutClass;
+
+  /// Current viewport width in logical pixels.
+  final double width;
+
+  /// Current viewport height in logical pixels.
+  final double height;
+
+  /// Returns the nearest [ViewportInfo] in the widget tree.
+  ///
+  /// Asserts that a [ViewportInfo] ancestor exists.
+  static ViewportInfo of(BuildContext context) {
+    final info = context.dependOnInheritedWidgetOfExactType<ViewportInfo>();
+    assert(info != null, 'No ViewportInfo found in widget tree');
+    return info!;
+  }
+
+  /// Whether the current layout is compact (< 600dp).
+  bool get isCompact => layoutClass == LayoutClass.compact;
+
+  /// Whether the current layout is medium (600–1023dp).
+  bool get isMedium => layoutClass == LayoutClass.medium;
+
+  /// Whether the current layout is expanded (≥ 1024dp).
+  bool get isExpanded => layoutClass == LayoutClass.expanded;
+
+  /// Whether the layout has room for at least two panels (medium or expanded).
+  bool get isWide => !isCompact;
+
+  @override
+  bool updateShouldNotify(ViewportInfo old) =>
+      layoutClass != old.layoutClass ||
+      width != old.width ||
+      height != old.height;
+}