feat(reservations): add price field with automatic budget entry creation

- Optional price and budget category fields on the reservation form
- When a price is set, a budget entry is automatically created on save
- Price and category stored in reservation metadata for reference
- Hint text shown when price is entered
- i18n keys for EN and DE

client/src/i18n/translations/de.ts

@@ -935,6 +935,10 @@ const de: Record<string, string | { name: string; category: string }[]> = {
   'reservations.linkAssignment': 'Mit Tagesplanung verknüpfen',
   'reservations.pickAssignment': 'Zuordnung aus dem Plan wählen...',
   'reservations.noAssignment': 'Keine Verknüpfung',
+  'reservations.price': 'Preis',
+  'reservations.budgetCategory': 'Budgetkategorie',
+  'reservations.budgetCategoryPlaceholder': 'z.B. Transport, Unterkunft',
+  'reservations.budgetHint': 'Beim Speichern wird automatisch ein Budgeteintrag erstellt.',
   'reservations.departureDate': 'Abflug',
   'reservations.arrivalDate': 'Ankunft',
   'reservations.departureTime': 'Abflugzeit',

client/src/i18n/translations/en.ts

@@ -932,6 +932,10 @@ const en: Record<string, string | { name: string; category: string }[]> = {
   'reservations.linkAssignment': 'Link to day assignment',
   'reservations.pickAssignment': 'Select an assignment from your plan...',
   'reservations.noAssignment': 'No link (standalone)',
+  'reservations.price': 'Price',
+  'reservations.budgetCategory': 'Budget category',
+  'reservations.budgetCategoryPlaceholder': 'e.g. Transport, Accommodation',
+  'reservations.budgetHint': 'A budget entry will be created automatically when saving.',
   'reservations.departureDate': 'Departure',
   'reservations.arrivalDate': 'Arrival',
   'reservations.departureTime': 'Dep. time',