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/hooks/useChatOverlay.test.ts

@@ -369,6 +369,37 @@ describe("useChatOverlay", () => {
     });
   });
 
+  describe("minimize/toggleMinimize guards", () => {
+    it("should not allow minimize when overlay is closed", () => {
+      const { result } = renderHook(() => useChatOverlay());
+
+      // Overlay starts closed
+      expect(result.current.isOpen).toBe(false);
+
+      act(() => {
+        result.current.minimize();
+      });
+
+      // Should remain unchanged - cannot minimize when closed
+      expect(result.current.isOpen).toBe(false);
+      expect(result.current.isMinimized).toBe(false);
+    });
+
+    it("should not allow toggleMinimize when overlay is closed", () => {
+      const { result } = renderHook(() => useChatOverlay());
+
+      expect(result.current.isOpen).toBe(false);
+
+      act(() => {
+        result.current.toggleMinimize();
+      });
+
+      // Should remain unchanged - cannot toggle minimize when closed
+      expect(result.current.isOpen).toBe(false);
+      expect(result.current.isMinimized).toBe(false);
+    });
+  });
+
   describe("storage error handling", () => {
     it("should call onStorageError when localStorage save fails", () => {
       const onStorageError = vi.fn();