feat: add auto/system color scheme option

- Extend ColorScheme type to include 'auto' alongside 'light' and 'dark'
- Allow 'auto' to persist to DB and cookie (remove the explicit guard)
- Server default and new-user default both changed to 'auto'
- layout.tsx: omit inline backgroundColor when scheme is 'auto'
- Avatar menu: replace toggle item with a Combobox (Auto/Light/Dark)
  mirroring the existing language selector pattern
- Appearance settings form and color scheme comboboxes updated with
  IconDeviceDesktop for the new auto option
- Spotlight command handles 'auto' state via useComputedColorScheme
- Old import: map legacy 'environment' value to 'auto'
- Add translation keys for 'auto' in all three relevant scopes

packages/server-settings/src/index.ts

@@ -36,7 +36,7 @@ export const defaultServerSettings = {
     enableGravatar: true,
   },
   appearance: {
-    defaultColorScheme: "light" as ColorScheme,
+    defaultColorScheme: "auto" as ColorScheme,
   },
   culture: {
     defaultLocale: "en" as SupportedLanguage,