feat(chat): add voice playback with auto-play and visual effects (#396)

## Features

### Core Functionality
- **Web Speech API Integration**: Add TTS (text-to-speech) playback for assistant messages
- **Manual Playback**: Click-to-play button next to each assistant message (🔊 icon)
- **Auto-play Mode**: Toggle switch in input bar to auto-play responses
- **Playback Controls**: Play/pause/stop with visual feedback

### User Interface
- **Playback Button**: Hover-activated button in message meta area (next to copy button)
- **Auto-play Switch**: Voice icon toggle in input top bar with state persistence
- **Mobile Optimization**: Buttons always visible on mobile (≤768px width)
- **Visual Feedback**:
  - Rainbow glowing border during playback (2px border, 10px/20px glow)
  - 4-second animation cycle through 6 colors
  - Play/pause icon toggle

### Voice Customization
- **Pitch/Rate Control**: Low-pitched (0.5) fast-speaking (1.2) "male voice"
- **Auto Voice Selection**: Attempts to select male voices across platforms (macOS: Yaoyao, Windows: David/Daniel)
- **Platform Compatibility**: Works with system-provided voices on macOS, iOS, Android, Windows

### Content Filtering
- Smart text extraction: filters code blocks, `<thinking>` tags, HTML
- Only assistant messages are eligible for playback
- Tool and system messages are excluded

### Internationalization
- Added 8 language translations (en, zh, de, es, fr, ja, ko, pt)
- New keys: `playSpeech`, `pauseSpeech`, `resumeSpeech`, `stopSpeech`, `autoPlaySpeech`, `speechNotSupported`

## Technical Details

### New Files
- `packages/client/src/composables/useSpeech.ts`: Core speech synthesis composable
  - Voice loading and selection logic
  - Single-instance global speech manager
  - Event handling (onstart, onend, onerror, onboundary)
  - State management (isPlaying, isPaused, currentMessageId)

### Modified Components
- **ChatInput.vue**: Auto-play switch with localStorage persistence
- **MessageItem.vue**:
  - Playback button integration
  - Event listener for auto-play triggers
  - Mobile-first button visibility
  - Rainbow border animation during playback

### Store Changes
- `chat.ts`:
  - Added `autoPlaySpeechEnabled` state
  - `setAutoPlaySpeech()` method
  - `playMessageSpeech()` method for event-based playback
  - Auto-play trigger on `run.completed` event

## Browser Support
- Requires Web Speech API support (all modern browsers)
- Graceful degradation: button hidden if API not supported
- Voice availability varies by platform and OS

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/client/src/i18n/locales/en.ts

@@ -111,6 +111,7 @@ export default {
     emptyState: 'Start a conversation with Hermes Agent',
     inputPlaceholder: 'Type a message... (Enter to send, Shift+Enter for new line)',
     attachFiles: 'Attach files',
+    autoPlaySpeech: 'Auto-play voice',
     stop: 'Stop',
     start: 'Start',
     stopGateway: 'Stop Gateway',
@@ -176,6 +177,11 @@ export default {
     copyBubble: 'Copy message',
     copiedBubble: 'Message copied',
     copyFailed: 'Copy failed',
+    playSpeech: 'Play voice',
+    pauseSpeech: 'Pause',
+    resumeSpeech: 'Resume',
+    stopSpeech: 'Stop',
+    speechNotSupported: 'Voice playback not supported in this browser',
   },
 
   // Jobs