feat: add web gateway — browser-based chat UI over WebSocket

New platform adapter that serves a full-featured chat interface via HTTP.
Enables access from any device on the network (phone, tablet, desktop).

Features:
- aiohttp server with WebSocket real-time messaging
- Token-based authentication
- Markdown rendering (marked.js) + code highlighting (highlight.js)
- Voice recording via MediaRecorder API + STT transcription
- Image, voice, and document display
- Typing indicator + message editing (streaming support)
- Mobile responsive dark theme
- Auto-reconnect on disconnect
- Media file cleanup (24h TTL)

Config: WEB_UI_ENABLED=true, WEB_UI_PORT=8765, WEB_UI_TOKEN=<token>
No new dependencies — uses aiohttp already in [messaging] extra.

gateway/run.py

@@ -829,6 +829,13 @@ class GatewayRunner:
                 return None
             return EmailAdapter(config)
 
+        elif platform == Platform.WEB:
+            from gateway.platforms.web import WebAdapter, check_web_requirements
+            if not check_web_requirements():
+                logger.warning("Web: aiohttp not installed. Run: pip install aiohttp")
+                return None
+            return WebAdapter(config)
+
         return None
     
     def _is_user_authorized(self, source: SessionSource) -> bool:
@@ -848,6 +855,11 @@ class GatewayRunner:
         if source.platform == Platform.HOMEASSISTANT:
             return True
 
+        # Web UI users are authenticated via token at the WebSocket level.
+        # No additional allowlist check needed.
+        if source.platform == Platform.WEB:
+            return True
+
         user_id = source.user_id
         if not user_id:
             return False