fix: shift edit time slot dropdown loading state and test coverage

The time slot dropdown in the shift edit dialog could flash the
"create a time slot first" alert during loading, and show raw ULIDs
when time slot data hadn't loaded yet. Fixed by:
- Adding loading state indicator to the time slot dropdown
- Using the shift's existing time_slot object as a fallback item
  while the full list is fetching
- Showing the dropdown (with loading spinner) instead of the
  misleading "no time slots" alert during fetch

Added test coverage for time_slot_id validation on shift updates:
- Update with valid same-event time slot (200)
- Update with cross-org time slot (422)
- Update on sub-event with parent festival time slot (200)
- Store/update responses include nested time_slot object

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/tests/Feature/Event/FestivalEventTest.php

@@ -477,6 +477,38 @@ class FestivalEventTest extends TestCase
         ]);
     }
 
+    public function test_update_shift_on_sub_event_with_parent_festival_time_slot(): void
+    {
+        $section = FestivalSection::factory()->create([
+            'event_id' => $this->subEvent->id,
+        ]);
+
+        $subEventTimeSlot = TimeSlot::factory()->create([
+            'event_id' => $this->subEvent->id,
+        ]);
+
+        $festivalTimeSlot = TimeSlot::factory()->create([
+            'event_id' => $this->festival->id,
+        ]);
+
+        // Create a shift with the sub-event time slot
+        $shift = Shift::factory()->create([
+            'festival_section_id' => $section->id,
+            'time_slot_id' => $subEventTimeSlot->id,
+        ]);
+
+        Sanctum::actingAs($this->orgAdmin);
+
+        // Update to use a festival time slot — should succeed
+        $response = $this->putJson("/api/v1/organisations/{$this->organisation->id}/events/{$this->subEvent->id}/sections/{$section->id}/shifts/{$shift->id}", [
+            'time_slot_id' => $festivalTimeSlot->id,
+        ]);
+
+        $response->assertOk()
+            ->assertJsonPath('data.time_slot_id', $festivalTimeSlot->id)
+            ->assertJsonPath('data.time_slot.id', $festivalTimeSlot->id);
+    }
+
     public function test_create_shift_on_local_section_with_other_event_time_slot_returns_422(): void
     {
         $section = FestivalSection::factory()->create([