feat: smart assign person dialog with conflict details and assignable-persons endpoint

Add GET /events/{event}/shifts/{shift}/assignable-persons endpoint that
returns approved persons with availability status, conflict details, and
already-assigned flags. Improve ShiftAssignmentService conflict errors to
include section name, time slot, and time range. Replace both assign
dialogs with a new AssignPersonDialog featuring search, crowd type
filtering, availability toggle, and inline conflict warnings.

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

api/app/Services/ShiftAssignmentService.php

@@ -24,7 +24,7 @@ final class ShiftAssignmentService
             $this->validateShiftIsOpen($shift);
             $this->validatePersonApproved($person);
             $this->validateClaimCapacity($shift);
-            $this->validateNoConflict($shift, $person);
+            $this->validateNoConflict($shift, $person, isClaim: true);
 
             $autoApprove = $shift->festivalSection->crew_auto_accepts;
             $status = $autoApprove
@@ -323,20 +323,30 @@ final class ShiftAssignmentService
     /**
      * @throws ValidationException
      */
-    private function validateNoConflict(Shift $shift, Person $person): void
+    private function validateNoConflict(Shift $shift, Person $person, bool $isClaim = false): void
     {
         if ($shift->allow_overlap) {
             return;
         }
 
-        $conflict = ShiftAssignment::where('person_id', $person->id)
+        $existingAssignment = ShiftAssignment::where('person_id', $person->id)
             ->where('time_slot_id', $shift->time_slot_id)
             ->active()
-            ->exists();
+            ->with(['shift.festivalSection', 'shift.timeSlot'])
+            ->first();
+
+        if ($existingAssignment) {
+            $section = $existingAssignment->shift->festivalSection->name;
+            $timeSlot = $existingAssignment->shift->timeSlot->name;
+            $time = $existingAssignment->shift->timeSlot->start_time
+                . '–' . $existingAssignment->shift->timeSlot->end_time;
+
+            $message = $isClaim
+                ? "Je bent al ingepland bij \"{$section}\" voor {$timeSlot} ({$time})."
+                : "Deze persoon is al ingepland bij \"{$section}\" voor {$timeSlot} ({$time}).";
 
-        if ($conflict) {
             throw ValidationException::withMessages([
-                'person_id' => ['Deze persoon is al ingepland voor dit tijdslot.'],
+                'person_id' => [$message],
             ]);
         }
     }