fix(ui): address PR #30 review findings

- ViewportInfo.updateShouldNotify: compare layoutClass only
  (not width/height), avoiding unnecessary rebuilds on every
  resize frame within the same layout class.
- ChatPanel: use BorderDirectional(start:) for RTL support.
- ChatPanel: localize 'Close chat' tooltip via AppL10n.chatCloseAction.
- Inline panel snackbar: localize via AppL10n.chatPanelCollapsedHint.
  New en/zh ARB entries added.
- _saveCurrentDraft(): removed — it was a self-assignment no-op.
  Draft persistence relies on ChatDetailView's didUpdateWidget
  (fires onDraftChanged on target switch) and dispose (fires on
  panel tear-down), both of which already populate _chatDrafts
  correctly without an explicit save call.
- _handleInlineChatViewport layout snackbar: use AppL10n.
- Audio level-meter: switch from callback-count (% 3) to time-based
  gating (std::time::Duration::from_millis(33)), robust to cpal
  buffer-size or sample-rate changes. Remove level_decimation_counter.
- chat_panel_test.dart: add AppL10n.localizationsDelegates so the
  test resolves l10n keys.

Tests: 183 passed, 2 skipped. Dart analyze clean.
cargo test -p chanora_audio --lib: 125 passed.

apps/chanora_flutter/lib/design/viewport_info.dart

@@ -57,8 +57,11 @@ class ViewportInfo extends InheritedWidget {
   bool get isWide => !isCompact;
 
   @override
-  bool updateShouldNotify(ViewportInfo old) =>
-      layoutClass != old.layoutClass ||
-      width != old.width ||
-      height != old.height;
+  bool updateShouldNotify(ViewportInfo old) => layoutClass != old.layoutClass;
+  // NOTE: width/height changes within the same layout class do NOT trigger
+  // notification. Dependents who genuinely need pixel-level dimensions
+  // (rare — most layouts should switch on layoutClass) must use a local
+  // LayoutBuilder. Notifying on every pixel would rebuild every dependent
+  // on every resize frame, which is the exact pessimisation this
+  // InheritedWidget exists to avoid.
 }