v2.1.0 — Real-time collaboration, performance & security overhaul

Real-Time Collaboration (WebSocket):
- WebSocket server with JWT auth and trip-based rooms
- Live sync for all CRUD operations (places, assignments, days, notes, budget, packing, reservations, files)
- Socket-based exclusion to prevent duplicate updates
- Auto-reconnect with exponential backoff
- Assignment move sync between days

Performance:
- 16 database indexes on all foreign key columns
- N+1 query fix in places, assignments and days endpoints
- Marker clustering (react-leaflet-cluster) with configurable radius
- List virtualization (react-window) for places sidebar
- useMemo for filtered places
- SQLite WAL mode + busy_timeout for concurrent writes
- Weather API: server-side cache (1h forecast, 15min current) + client sessionStorage
- Google Places photos: persisted to DB after first fetch
- Google Details: 3-tier cache (memory → sessionStorage → API)

Security:
- CORS auto-configuration (production: same-origin, dev: open)
- API keys removed from /auth/me response
- Admin-only endpoint for reading API keys
- Path traversal prevention in cover image deletion
- JWT secret persisted to file (survives restarts)
- Avatar upload file extension whitelist
- API key fallback: normal users use admin's key without exposure
- Case-insensitive email login

Dark Mode:
- Fixed hardcoded colors across PackingList, Budget, ReservationModal, ReservationsPanel
- Mobile map buttons and sidebar sheets respect dark mode
- Cluster markers always dark

UI/UX:
- Redesigned login page with animated planes, stars and feature cards
- Admin: create user functionality with CustomSelect
- Mobile: day-picker popup for assigning places to days
- Mobile: touch-friendly reorder buttons (32px targets)
- Mobile: responsive text (shorter labels on small screens)
- Packing list: index-based category colors
- i18n: translated date picker placeholder, fixed German labels
- Default map tile: CartoDB Light

client/src/components/shared/CustomDateTimePicker.jsx

@@ -8,7 +8,7 @@ function getWeekday(year, month, day) { return new Date(year, month, day).getDay
 
 // ── Datum-Only Picker ────────────────────────────────────────────────────────
 export function CustomDatePicker({ value, onChange, placeholder, style = {} }) {
-  const { locale } = useTranslation()
+  const { locale, t } = useTranslation()
   const [open, setOpen] = useState(false)
   const ref = useRef(null)
   const dropRef = useRef(null)
@@ -67,7 +67,7 @@ export function CustomDatePicker({ value, onChange, placeholder, style = {} }) {
         onMouseEnter={e => e.currentTarget.style.borderColor = 'var(--text-faint)'}
         onMouseLeave={e => { if (!open) e.currentTarget.style.borderColor = 'var(--border-primary)' }}>
         <Calendar size={14} style={{ color: 'var(--text-faint)', flexShrink: 0 }} />
-        <span>{displayValue || placeholder || 'Datum'}</span>
+        <span>{displayValue || placeholder || t('common.date')}</span>
       </button>
 
       {open && ReactDOM.createPortal(