fix(frontend): a11y on clickable rows + correct c2 source field + pill metric alignment (sprint 8 design-review)

F1: add tabIndex/role/onKeyDown/aria-expanded to C2TasksPanel expander rows and
    C2CallbackPicker callback rows; focus-visible ring via Tailwind utilities
F2: add source:'mimic'|'import' to C2TaskListItem; C2TasksPanel reads task.source
    instead of mapping_applied for the Source badge label
F3: align C2TaskStatusBadge and C2CallbackPicker Active/Inactive pill metrics to
    py-[6px] text-[14px] font-medium (matches SimulationStatusBadge / StatusBadge)
F4: replace hand-rolled Source pill class string with badge-pill-outline recipe
Tests: 212/212 passing (+3 new: Enter/Space key on expander, Enter key on callback row)

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

frontend/tests/components/ExecuteViaC2Modal.test.tsx

@@ -157,4 +157,15 @@ describe('ExecuteViaC2Modal', () => {
     const textarea = screen.getByTestId('c2-commands-textarea') as HTMLTextAreaElement;
     expect(textarea.value).toBe('net user\nwhoami /all');
   });
+
+  it('Enter key on callback row selects it (a11y)', async () => {
+    renderModal('whoami');
+    await waitFor(() => {
+      expect(screen.getAllByTestId('c2-callback-row')).toHaveLength(2);
+    });
+    const firstRow = screen.getAllByTestId('c2-callback-row')[0];
+    fireEvent.keyDown(firstRow, { key: 'Enter' });
+    // Row is now selected → Launch button should be enabled
+    expect(screen.getByTestId('c2-launch-btn')).not.toBeDisabled();
+  });
 });