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.

WHATS_NEW.md

@@ -42,6 +42,10 @@ Within each section, order by **user impact** (most noticeable first) — not PR
 
 - Turn on **Remember EQ per device** under **Settings → Audio** and Psysonic keeps a separate equalizer setup for each output — speakers, headphones, a USB DAC — and switches to the right one automatically when you change devices. Off by default.
 
+### Themes — follow your system's light and dark mode
+
+- The theme scheduler can now match your **system's light/dark setting** instead of a fixed clock: pick a light theme and a dark one, and Psysonic switches along with your OS. Choose **Time of Day** or **System Theme** under **Settings → Themes** — the existing time-based schedule is still there.
+
 ## Fixed
 
 ### Playback and audio