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/old-import/src/mappers/map-user.ts

@@ -24,7 +24,7 @@ export const mapAndDecryptUsers = (importUsers: OldmarrImportUser[], encryptionT
       oldId: id,
       id: createId(),
       name: user.name.toLowerCase(),
-      colorScheme: settings?.colorScheme === "environment" ? undefined : settings?.colorScheme,
+      colorScheme: settings?.colorScheme === "environment" ? "auto" : settings?.colorScheme,
       firstDayOfWeek: settings?.firstDayOfWeek === "sunday" ? 0 : settings?.firstDayOfWeek === "monday" ? 1 : 6,
       provider: "credentials",
       pingIconsEnabled: settings?.replacePingWithIcons,