fix(HAI-048): fix checkbox input styling in Settings modal

- Exclude checkbox inputs from full-width text input styles using :not([type="checkbox"])
- Add dedicated checkbox styling with proper size, accent color, and cursor
- Add test verifying groupOverlappingFiles input renders as a checkbox

packages/dashboard/app/components/__tests__/SettingsModal.test.tsx

@@ -86,6 +86,15 @@ describe("SettingsModal", () => {
     expect(screen.getByText("Auto-merge completed tasks")).toBeTruthy();
   });
 
+  it("groupOverlappingFiles input has type checkbox", async () => {
+    render(<SettingsModal onClose={onClose} addToast={addToast} />);
+    await waitFor(() => expect(fetchSettings).toHaveBeenCalled());
+
+    const checkbox = screen.getByLabelText("Serialize tasks with overlapping files");
+    expect(checkbox).toBeTruthy();
+    expect(checkbox.getAttribute("type")).toBe("checkbox");
+  });
+
   it("save button calls updateSettings with form data", async () => {
     render(<SettingsModal onClose={onClose} addToast={addToast} />);
     await waitFor(() => expect(fetchSettings).toHaveBeenCalled());

packages/dashboard/app/styles.css

@@ -353,7 +353,7 @@ html, body {
 }
 .optional { font-weight: 400; text-transform: none; letter-spacing: 0; }
 
-.form-group input,
+.form-group input:not([type="checkbox"]),
 .form-group textarea {
   width: 100%;
   padding: 8px 12px;
@@ -366,10 +366,18 @@ html, body {
   outline: none;
   transition: border-color 0.15s;
 }
-.form-group input:focus,
+.form-group input:not([type="checkbox"]):focus,
 .form-group textarea:focus {
   border-color: var(--todo);
 }
+.form-group input[type="checkbox"] {
+  width: 16px;
+  height: 16px;
+  padding: 0;
+  margin: 0;
+  accent-color: var(--todo);
+  cursor: pointer;
+}
 .form-group textarea { resize: vertical; }
 
 /* === Settings Layout === */