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/SimulationFormPage.test.tsx

@@ -388,6 +388,7 @@ describe('SimulationFormPage — C2 tasks panel visibility', () => {
           completed: true,
           output: 'SYSTEM',
           mapping_applied: false,
+          source: 'import',
           created_at: '2026-06-10T10:00:00',
           completed_at: '2026-06-10T10:00:05',
         },