fix(web): Address review findings for M4-LLM integration

- Sanitize user-facing error messages (no raw API/DB errors)
- Remove dead try/catch from Chat.tsx handleSendMessage
- Add onError callback for persistence errors in useChat
- Add console.error logging to loadConversation
- Guard minimize/toggleMinimize against closed overlay state
- Improve error dedup bucketing for non-DOMException errors
- Add tests: non-Error throws, updateConversation failure,
  minimize/toggleMinimize guards

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

apps/web/src/components/chat/Chat.tsx

@@ -73,9 +73,6 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
   } = useChat({
     model: "llama3.2",
     ...(initialProjectId !== undefined && { projectId: initialProjectId }),
-    onError: (_err) => {
-      // Error is handled by the useChat hook's state
-    },
   });
 
   // Connect to WebSocket for real-time updates (when we have a user)
@@ -180,11 +177,7 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
 
   const handleSendMessage = useCallback(
     async (content: string) => {
-      try {
-        await sendMessage(content);
-      } catch (err) {
-        console.error("Error sending message:", err);
-      }
+      await sendMessage(content);
     },
     [sendMessage]
   );