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/app/Services/ShiftAssignmentService.php

@@ -168,18 +168,26 @@ final class ShiftAssignmentService
             $this->validateStatusTransition($assignment, ShiftAssignmentStatus::APPROVED);
 
             $shift = $assignment->shift;
+            $oldStatus = $assignment->status;
+
+            // Log overbooking for audit trail (organisers may intentionally overbook)
             $approvedCount = $shift->shiftAssignments()
                 ->where('status', ShiftAssignmentStatus::APPROVED)
                 ->count();
 
             if ($approvedCount >= $shift->slots_total) {
-                throw ValidationException::withMessages([
-                    'shift' => ['Shift is vol — alle slots zijn bezet.'],
-                ]);
+                activity('shift_assignment')
+                    ->causedBy($approvedBy)
+                    ->performedOn($shift)
+                    ->withProperties([
+                        'filled_slots' => $approvedCount,
+                        'slots_total' => $shift->slots_total,
+                        'assignment_id' => $assignment->id,
+                        'person_id' => $assignment->person_id,
+                    ])
+                    ->log('shift.overbooked_approval');
             }
 
-            $oldStatus = $assignment->status;
-
             $assignment->status = ShiftAssignmentStatus::APPROVED;
             $assignment->approved_by = $approvedBy->id;
             $assignment->approved_at = now();
@@ -280,16 +288,22 @@ final class ShiftAssignmentService
                 }
 
                 $shift = $assignment->shift;
+
+                // Log overbooking for audit trail
                 $approvedCount = $shift->shiftAssignments()
                     ->where('status', ShiftAssignmentStatus::APPROVED)
                     ->count();
 
                 if ($approvedCount >= $shift->slots_total) {
-                    return [
-                        'assignment_id' => $assignment->id,
-                        'status' => 'skipped',
-                        'reason' => 'Shift is vol.',
-                    ];
+                    activity('shift_assignment')
+                        ->causedBy($approvedBy)
+                        ->performedOn($shift)
+                        ->withProperties([
+                            'filled_slots' => $approvedCount,
+                            'slots_total' => $shift->slots_total,
+                            'assignment_id' => $assignment->id,
+                        ])
+                        ->log('shift.overbooked_approval');
                 }
 
                 $assignment->status = ShiftAssignmentStatus::APPROVED;