feat: fix time slot hierarchy — seeder, API include_children, frontend dropdown, navigation

Restructure the festival hierarchy end-to-end:

Seeder: Remove duplicate festival-level VOLUNTEER time slots, keep only CREW
operational slots. Rename sub-events to "Dag 1/2/3 — ..." pattern. Change
Nachtsecurity to Security (cross_event). EHBO/Security shifts now use sub-event
time slots via cross_event exception. Add flat event "Braderie Dorpstown 2026".

API: Add ?include_children=true to TimeSlotController for festivals, returning
all sub-event time slots with source and event_name fields. Update
StoreShiftRequest and UpdateShiftRequest to accept child time slots for
cross_event sections.

Frontend: Create useTimeSlotDropdown composable with 4-scenario dropdown logic.
Replace AppSelect with VAutocomplete in CreateShiftDialog with grouped items,
dimmed festival slots, and info tooltips. Add InfoTooltip reusable component.
Show festival context labels on cross_event sections in sub-event section lists.
Add read-only festival time slots on sub-event time-slots page. Add cross_event
context banner with "Bekijk alle diensten" link.

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

apps/app/components.d.ts

@@ -65,6 +65,7 @@ declare module 'vue' {
     EventTabsNav: typeof import('./src/components/events/EventTabsNav.vue')['default']
     I18n: typeof import('./src/@core/components/I18n.vue')['default']
     ImportFromEventDialog: typeof import('./src/components/event/ImportFromEventDialog.vue')['default']
+    InfoTooltip: typeof import('./src/components/common/InfoTooltip.vue')['default']
     InviteMemberDialog: typeof import('./src/components/members/InviteMemberDialog.vue')['default']
     MoreBtn: typeof import('./src/@core/components/MoreBtn.vue')['default']
     Notifications: typeof import('./src/@core/components/Notifications.vue')['default']