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/src/components/C2CallbackPicker.tsx

@@ -57,14 +57,22 @@ export function C2CallbackPicker({
                 key={cb.display_id}
                 data-testid={rowTestId}
                 onClick={() => onSelect(cb.display_id)}
-                className={`cursor-pointer border-b border-hairline ${
+                onKeyDown={(e) => {
+                  if (e.key === 'Enter' || e.key === ' ') {
+                    e.preventDefault();
+                    onSelect(cb.display_id);
+                  }
+                }}
+                tabIndex={0}
+                role="button"
+                className={`cursor-pointer border-b border-hairline focus:outline-none focus-visible:ring-2 focus-visible:ring-primary ${
                   isSelected ? 'bg-primary-soft' : 'hover:bg-cloud'
                 }`}
               >
                 <td className="px-md py-sm font-mono">{cb.display_id}</td>
                 <td className="px-md py-sm">
                   <span
-                    className={`inline-flex items-center rounded-pill px-3 py-[4px] text-[12px] leading-[1.3] font-bold ${
+                    className={`inline-flex items-center rounded-pill px-3 py-[6px] text-[14px] leading-[1.3] font-medium ${
                       cb.active
                         ? 'bg-primary-soft text-primary-deep'
                         : 'bg-cloud text-graphite border border-hairline'