fix(frontend): restore member self-assignment for shifts and tasks

Root Cause:
- Shift: Next.js 16.1.6 incompatible rewrite pattern caused runtime SyntaxError
- Task: Missing self-assignment UI for member role

Fix:
- Updated next.config.ts rewrite pattern from regex to wildcard syntax
- Added "Assign to Me" button to task detail page with useSession integration
- Added test coverage for self-assignment behavior with session mocks

Testing:
- Lint: ✅ PASS (ESLint v9)
- Tests: ✅ 47/47 PASS (Vitest v4.0.18)
- Build: ✅ PASS (Next.js 16.1.6, 12 routes)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/components/__tests__/task-detail.test.tsx

@@ -11,6 +11,13 @@ vi.mock('next/navigation', () => ({
   })),
 }));
 
+vi.mock('next-auth/react', () => ({
+  useSession: vi.fn(() => ({
+    data: { user: { id: 'user-123' } },
+    status: 'authenticated',
+  })),
+}));
+
 vi.mock('@/hooks/useTasks', () => ({
   useTask: vi.fn(),
   useUpdateTask: vi.fn(),
@@ -74,4 +81,63 @@ describe('TaskDetailPage', () => {
     expect(screen.getByText('Mark as Done')).toBeInTheDocument();
     expect(screen.getByText('Back to InProgress')).toBeInTheDocument();
   });
+
+  it('renders Assign to Me button when task unassigned and session exists', async () => {
+    (useTask as ReturnType<typeof vi.fn>).mockReturnValue({
+      data: { 
+        id: '1', 
+        title: 'Task 1', 
+        status: 'Open', 
+        assigneeId: null,
+        description: 'Desc', 
+        createdAt: '2024-01-01', 
+        updatedAt: '2024-01-01' 
+      },
+      isLoading: false,
+      error: null,
+    });
+
+    const params = Promise.resolve({ id: '1' });
+    await act(async () => {
+      render(<TaskDetailPage params={params} />);
+    });
+    
+    expect(screen.getByText('Assign to Me')).toBeInTheDocument();
+  });
+
+  it('calls updateTask with assigneeId when Assign to Me clicked', async () => {
+    const mockMutate = vi.fn();
+    (useTask as ReturnType<typeof vi.fn>).mockReturnValue({
+      data: { 
+        id: '1', 
+        title: 'Task 1', 
+        status: 'Open', 
+        assigneeId: null,
+        description: 'Desc', 
+        createdAt: '2024-01-01', 
+        updatedAt: '2024-01-01' 
+      },
+      isLoading: false,
+      error: null,
+    });
+    (useUpdateTask as ReturnType<typeof vi.fn>).mockReturnValue({
+      mutate: mockMutate,
+      isPending: false,
+    });
+
+    const params = Promise.resolve({ id: '1' });
+    await act(async () => {
+      render(<TaskDetailPage params={params} />);
+    });
+
+    const button = screen.getByText('Assign to Me');
+    await act(async () => {
+      button.click();
+    });
+
+    expect(mockMutate).toHaveBeenCalledWith({
+      id: '1',
+      data: { assigneeId: 'user-123' },
+    });
+  });
 });