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>

dev-docs/API.md

@@ -180,6 +180,46 @@ Auth: org_admin or event_manager on the event's organisation.
 - `POST /events/{event}/shift-assignments/{shiftAssignment}/reject` — reject pending assignment
 - `POST /events/{event}/shift-assignments/{shiftAssignment}/cancel` — cancel assignment
 - `POST /events/{event}/shift-assignments/bulk-approve` — bulk approve multiple assignments
+- `GET /events/{event}/shifts/{shift}/assignable-persons` — list approved persons with availability status
+
+### Assignable Persons
+
+`GET /events/{event}/shifts/{shift}/assignable-persons`
+
+Returns all approved persons for the event with availability status for this shift's time slot.
+Persons are sorted: available first, then unavailable (conflict), then already assigned.
+
+```json
+{
+  "data": [
+    {
+      "id": "ulid",
+      "name": "Jan de Vries",
+      "email": "jan@gmail.com",
+      "status": "approved",
+      "crowd_type": { "id": "ulid", "name": "Vrijwilliger", "system_type": "VOLUNTEER" },
+      "is_available": true,
+      "already_assigned": false,
+      "conflict": null
+    },
+    {
+      "id": "ulid",
+      "name": "Ahmed Hassan",
+      "email": "ahmed.h@gmail.com",
+      "status": "approved",
+      "crowd_type": { "id": "ulid", "name": "Vrijwilliger", "system_type": "VOLUNTEER" },
+      "is_available": false,
+      "already_assigned": false,
+      "conflict": {
+        "section_name": "EHBO",
+        "shift_title": "EHBO Post",
+        "time_slot_name": "Zaterdag Dag",
+        "time": "10:00–18:00"
+      }
+    }
+  ]
+}
+```
 
 ### Query Parameters (index)