feat(themes): add follow-system mode to the theme scheduler (#1163)

* feat(themes): add follow-system mode to the theme scheduler

The theme scheduler can now switch the day/night theme pair by the OS
light/dark preference instead of a clock schedule. A segmented control
picks the trigger; in system mode the time inputs are hidden and the two
theme pickers read as Light/Dark.

The OS theme is read via the native Tauri window theme API (theme() +
onThemeChanged) rather than the Web prefers-color-scheme media query,
which is unreliable through WebKitGTK on Linux. Live updates land
instantly where the platform forwards them; on Linux setups that don't,
a hint notes the change applies after an app restart.

* docs(changelog): add follow-system theme mode entry (#1163)

CHANGELOG Added entry, contributor credit and What's New highlight for the
theme scheduler's new system-theme mode.

src/locales/de/settings.ts

@@ -650,12 +650,18 @@ export const settings = {
   seekbarRetrotape: 'Retro-Band',
   themeSchedulerTitle: 'Theme-Zeitplan',
   themeSchedulerEnable: 'Theme-Zeitplan aktivieren',
-  themeSchedulerEnableSub: 'Wechselt automatisch zwischen zwei Themes basierend auf der Uhrzeit',
+  themeSchedulerEnableSub: 'Wechselt automatisch zwischen zwei Themes – nach Uhrzeit oder System-Theme',
   themeSchedulerDayTheme: 'Tages-Theme',
   themeSchedulerDayStart: 'Tag beginnt um',
   themeSchedulerNightTheme: 'Nacht-Theme',
   themeSchedulerNightStart: 'Nacht beginnt um',
   themeSchedulerActiveHint: 'Theme-Zeitplan ist aktiv - Themes werden automatisch gewechselt.',
+  themeSchedulerModeLabel: 'Wechseln nach',
+  themeSchedulerModeTime: 'Uhrzeit',
+  themeSchedulerModeSystem: 'System-Theme',
+  themeSchedulerLightTheme: 'Helles Theme',
+  themeSchedulerDarkTheme: 'Dunkles Theme',
+  themeSchedulerSystemRestartHint: 'Ein Wechsel des System-Designs (hell/dunkel) wird erst nach einem Neustart der App übernommen.',
   visualOptionsTitle: 'Visuelle Optionen',
   coverArtBackground: 'Cover-Hintergrund',
   coverArtBackgroundSub: 'Zeigt verschwommenes Cover als Hintergrund in Album/Playlist-Kopfzeilen',