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/Layout/Navbar.jsx

@@ -5,7 +5,7 @@ import { useAuthStore } from '../../store/authStore'
 import { useSettingsStore } from '../../store/settingsStore'
 import { useTranslation } from '../../i18n'
 import { addonsApi } from '../../api/client'
-import { Plane, LogOut, Settings, ChevronDown, Shield, ArrowLeft, Users, Moon, Sun, CalendarDays, Briefcase, Globe } from 'lucide-react'
+import { Plane, LogOut, Settings, ChevronDown, Shield, ArrowLeft, Users, Moon, Sun, Monitor, CalendarDays, Briefcase, Globe } from 'lucide-react'
 
 const ADDON_ICONS = { CalendarDays, Briefcase, Globe }
 
@@ -18,7 +18,8 @@ export default function Navbar({ tripTitle, tripId, onBack, showBack, onShare })
   const [userMenuOpen, setUserMenuOpen] = useState(false)
   const [appVersion, setAppVersion] = useState(null)
   const [globalAddons, setGlobalAddons] = useState([])
-  const dark = settings.dark_mode
+  const darkMode = settings.dark_mode
+  const dark = darkMode === true || darkMode === 'dark' || (darkMode === 'auto' && window.matchMedia('(prefers-color-scheme: dark)').matches)
 
   const loadAddons = () => {
     if (user) {
@@ -46,8 +47,8 @@ export default function Navbar({ tripTitle, tripId, onBack, showBack, onShare })
     navigate('/login')
   }
 
-  const toggleDark = () => {
-    updateSetting('dark_mode', !dark).catch(() => {})
+  const toggleDarkMode = () => {
+    updateSetting('dark_mode', dark ? 'light' : 'dark').catch(() => {})
   }
 
   return (
@@ -139,8 +140,8 @@ export default function Navbar({ tripTitle, tripId, onBack, showBack, onShare })
         </button>
       )}
 
-      {/* Dark mode toggle */}
-      <button onClick={toggleDark} title={dark ? t('nav.lightMode') : t('nav.darkMode')}
+      {/* Dark mode toggle (light ↔ dark, overrides auto) */}
+      <button onClick={toggleDarkMode} title={dark ? t('nav.lightMode') : t('nav.darkMode')}
         className="p-2 rounded-lg transition-colors flex-shrink-0"
         style={{ color: 'var(--text-muted)' }}
         onMouseEnter={e => e.currentTarget.style.background = 'var(--bg-hover)'}