fix: message sending fallback + iOS zoom on input + openOrCreatePrivate

- MessageInput: HTTP fallback when WebSocket not OPEN (prevents silent failures)
- WsClient: add trySend() returning bool to detect WS availability
- Backend: POST /api/messages/chat/:chatId endpoint for HTTP message send
- openOrCreatePrivate: use openChat() for consistency after refreshing chat list
- iOS fix: inputs get font-size:16px on mobile to prevent Safari auto-zoom

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/ws.ts

@@ -59,6 +59,14 @@ class WsClient {
     }
   }
 
+  trySend(type: string, payload: any): boolean {
+    if (this.ws?.readyState === WebSocket.OPEN) {
+      this.ws.send(JSON.stringify({ type, payload }));
+      return true;
+    }
+    return false;
+  }
+
   disconnect() {
     this.token = null;
     if (this.reconnectTimer) clearTimeout(this.reconnectTimer);