feat(settings): move language to its own section with scalable dropdown picker (#906)

- New Language row in the main Settings list with current-language summary
- New SettingsLanguage drill-down page with an OutlinedButton + DropdownMenu
  picker (scales to any number of languages; same pattern as theme presets)
- Shared supportedLanguages()/languageLabel() helpers — adding a language
  is now one entry in one place
- Removed the 4-option segmented pill row from the Appearance page
- Added settings_sec_language string (en + zh)

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/NavigationKeys.kt

@@ -75,6 +75,8 @@ import kotlinx.serialization.Serializable
 
 @Serializable data object SettingsAppearance : NavKey
 
+@Serializable data object SettingsLanguage : NavKey
+
 @Serializable data object SettingsChat : NavKey
 
 @Serializable data object SettingsBehavior : NavKey