fix: allow organiser to approve shift assignments when shift is full

The approve() and bulkApprove() methods in ShiftAssignmentService
hard-blocked with a 422 when all slots were filled. This was incorrect
for organiser actions — only volunteer claims (portal self-service)
should enforce capacity limits. Organiser assign() already allowed
overbooking, making the approve block inconsistent.

Changes:
- Remove capacity hard-block from approve() and bulkApprove(), replace
  with audit log entry (shift.overbooked_approval)
- Add overbook confirmation dialog in ShiftDetailPanel before approving
  a full shift (single + bulk approve)
- Add onError handlers to all mutations in ShiftDetailPanel (approve,
  reject, cancel, bulk-approve) so errors display in the snackbar
- Add global 422 validation error display in axios interceptor via
  useNotificationStore as safety net for all components
- Add PHPUnit test for approve-when-full scenario

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

api/tests/Feature/Api/V1/ShiftAssignmentWorkflowTest.php

@@ -406,6 +406,44 @@ class ShiftAssignmentWorkflowTest extends TestCase
             ->assertJsonPath('data.rejection_reason', 'Onvoldoende ervaring voor deze rol.');
     }
 
+    public function test_approve_allows_overbooking_when_shift_is_full(): void
+    {
+        $shift = $this->createOpenShift(['slots_total' => 1]);
+
+        // Fill the slot
+        ShiftAssignment::factory()->approved()->create([
+            'shift_id' => $shift->id,
+            'person_id' => Person::factory()->approved()->create([
+                'event_id' => $this->event->id,
+                'crowd_type_id' => $this->crowdType->id,
+            ])->id,
+            'time_slot_id' => $this->timeSlot->id,
+        ]);
+
+        // Pending assignment to approve
+        $assignment = ShiftAssignment::factory()->create([
+            'shift_id' => $shift->id,
+            'person_id' => $this->person->id,
+            'time_slot_id' => $this->timeSlot->id,
+            'status' => ShiftAssignmentStatus::PENDING_APPROVAL,
+        ]);
+
+        Sanctum::actingAs($this->orgAdmin);
+
+        $response = $this->postJson(
+            "/api/v1/organisations/{$this->organisation->id}/events/{$this->event->id}/shift-assignments/{$assignment->id}/approve",
+        );
+
+        $response->assertOk()
+            ->assertJsonPath('data.status', 'approved');
+
+        $this->assertDatabaseHas('shift_assignments', [
+            'id' => $assignment->id,
+            'status' => ShiftAssignmentStatus::APPROVED->value,
+            'approved_by' => $this->orgAdmin->id,
+        ]);
+    }
+
     public function test_cannot_approve_already_approved(): void
     {
         $shift = $this->createOpenShift();