v2.5.7: Reservation overhaul, Day Detail Panel, i18n, paste support, auto dark mode

BREAKING: Reservations have been completely rebuilt. Existing place-level
reservations are no longer used. All reservations must be re-created via
the Bookings tab. Your trips, places, and other data are unaffected.

Reservation System (rebuilt from scratch):
- Reservations now link to specific day assignments instead of places
- Same place on different days can have independent reservations
- New assignment picker in booking modal (grouped by day, searchable)
- Removed day/place dropdowns from booking form
- Reservation badges in day plan sidebar with type-specific icons
- Reservation details in place inspector (only for selected assignment)
- Reservation summary in day detail panel

Day Detail Panel (new):
- Opens on day click in the sidebar
- Detailed weather: hourly forecast, precipitation, wind, sunrise/sunset
- Historical climate averages for dates beyond 16 days
- Accommodation management with check-in/check-out, confirmation number
- Hotel assignment across multiple days with day range picker
- Reservation overview for the day

Places:
- Places can now be assigned to the same day multiple times
- Start time + end time fields (replaces single time field)
- Map badges show multiple position numbers (e.g. "1 · 4")
- Route optimization fixed for duplicate places
- File attachments during place editing (not just creation)
- Cover image upload during trip creation (not just editing)
- Paste support (Ctrl+V) for images in trip, place, and file forms

Internationalization:
- 200+ hardcoded German strings translated to i18n (EN + DE)
- Server error messages in English
- Category seeds in English for new installations
- All planner, register, photo, packing components translated

UI/UX:
- Auto dark mode (follows system preference, configurable in settings)
- Navbar toggle switches light/dark (overrides auto)
- Sidebar minimize buttons z-index fixed
- Transport mode selector removed from day plan
- CustomSelect supports grouped headers (isHeader option)
- Optimistic updates for day notes (instant feedback)
- Booking cards redesigned with type-colored headers and structured details

Weather:
- Wind speed in mph when using Fahrenheit setting
- Weather description language matches app language

Admin:
- Weather info panel replaces OpenWeatherMap key input
- "Recommended" badge styling updated

client/src/components/Admin/BackupPanel.jsx

@@ -387,7 +387,7 @@ export default function BackupPanel() {
               </div>
               <div>
                 <h3 style={{ margin: 0, fontSize: 16, fontWeight: 700, color: 'white' }}>
-                  {language === 'de' ? 'Backup wiederherstellen?' : 'Restore Backup?'}
+                  {t('backup.restoreConfirmTitle')}
                 </h3>
                 <p style={{ margin: '2px 0 0', fontSize: 12, color: 'rgba(255,255,255,0.8)' }}>
                   {restoreConfirm.filename}
@@ -398,17 +398,13 @@ export default function BackupPanel() {
             {/* Body */}
             <div style={{ padding: '20px 24px' }}>
               <p className="text-gray-700 dark:text-gray-300" style={{ fontSize: 13, lineHeight: 1.6, margin: 0 }}>
-                {language === 'de'
-                  ? 'Alle aktuellen Daten (Reisen, Orte, Benutzer, Uploads) werden unwiderruflich durch das Backup ersetzt. Dieser Vorgang kann nicht rückgängig gemacht werden.'
-                  : 'All current data (trips, places, users, uploads) will be permanently replaced by the backup. This action cannot be undone.'}
+                {t('backup.restoreWarning')}
               </p>
 
               <div style={{ marginTop: 14, padding: '10px 12px', borderRadius: 10, fontSize: 12, lineHeight: 1.5 }}
                 className="bg-red-50 dark:bg-red-900/30 text-red-700 dark:text-red-300 border border-red-200 dark:border-red-800"
               >
-                {language === 'de'
-                  ? 'Tipp: Erstelle zuerst ein Backup des aktuellen Stands, bevor du wiederherstellst.'
-                  : 'Tip: Create a backup of the current state before restoring.'}
+                {t('backup.restoreTip')}
               </div>
             </div>
 
@@ -419,7 +415,7 @@ export default function BackupPanel() {
                 className="text-gray-600 dark:text-gray-400 hover:bg-gray-100 dark:hover:bg-gray-700"
                 style={{ padding: '9px 20px', borderRadius: 10, fontSize: 13, fontWeight: 600, border: 'none', cursor: 'pointer', fontFamily: 'inherit' }}
               >
-                {language === 'de' ? 'Abbrechen' : 'Cancel'}
+                {t('common.cancel')}
               </button>
               <button
                 onClick={executeRestore}
@@ -427,7 +423,7 @@ export default function BackupPanel() {
                 onMouseEnter={e => e.currentTarget.style.background = '#b91c1c'}
                 onMouseLeave={e => e.currentTarget.style.background = '#dc2626'}
               >
-                {language === 'de' ? 'Ja, wiederherstellen' : 'Yes, restore'}
+                {t('backup.restoreConfirm')}
               </button>
             </div>
           </div>