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/config/settingsCredits.ts

@@ -383,6 +383,7 @@ const CONTRIBUTOR_ENTRIES = [
       'Artist artwork from fanart.tv — opt-in fullscreen background + artist-header banner, off by default (PR #1137)',
       'Per-device equalizer profiles — opt-in, remembers and restores the EQ for each audio output device (PR #1146)',
       'Hungarian (Magyar) translation (PR #1149)',
+      'Theme scheduler — follow the OS light/dark setting as an alternative to the time-based day/night schedule (PR #1163)',
     ],
   },
   {