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.test.tsx

@@ -176,22 +176,19 @@ describe("Chat", () => {
     });
   });
 
-  describe("sendMessage error handling", () => {
-    it("should log error when sendMessage fails", async () => {
-      const sendError = new Error("Send failed");
-      const mockSendMessage = vi.fn().mockRejectedValue(sendError);
+  describe("sendMessage delegation", () => {
+    it("should delegate to useChat.sendMessage without extra error handling", async () => {
+      const mockSendMessage = vi.fn().mockResolvedValue(undefined);
       mockUseChat.mockReturnValue(createMockUseChatReturn({ sendMessage: mockSendMessage }));
 
       const ref = createRef<ChatRef>();
       const { getByTestId } = render(<Chat ref={ref} />);
 
-      // Click the send button (which calls handleSendMessage)
       const sendButton = getByTestId("chat-input");
       sendButton.click();
 
-      // Wait for async handling
       await vi.waitFor(() => {
-        expect(consoleSpy).toHaveBeenCalledWith("Error sending message:", sendError);
+        expect(mockSendMessage).toHaveBeenCalledWith("test message");
       });
     });
   });