feat: in-app display language switcher (Settings → Appearance) (#604)

Add a Language selector to Settings → Appearance that overrides the app
display language independent of the device locale. Persists a BCP-47 code
in ServerStoreState.appLanguage; MainActivity.attachBaseContext applies it
via a ContextWrapper (works on minSdk 26, no AppCompat dependency needed
since the Activity is a plain ComponentActivity). Selecting a language
recreates the activity so resources re-resolve immediately.

Includes System default / English / 한국어 options, plus a unit test for
the locale-code resolver.

Note: the new string keys (settings_item_language, language_system,
language_english, language_korean) should be backfilled into values-ko
once PR #602 lands.

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

app/src/main/java/com/m57/hermescontrol/data/local/AuthManager.kt

@@ -498,6 +498,15 @@ object AuthManager {
         serverStore.update { it.copy(typingEffectEnabled = enabled) }
     }
 
+    // ── App display language ───────────────────────────────────────────
+    // "system" follows the device locale; any other value is a BCP-47 code.
+
+    fun getAppLanguage(): String = serverStore.getLatestState().appLanguage
+
+    fun setAppLanguage(code: String) {
+        serverStore.update { it.copy(appLanguage = code) }
+    }
+
     fun getTypingEffectDelayMs(): Int = serverStore.getLatestState().typingEffectDelayMs
 
     fun setTypingEffectDelayMs(delayMs: Int) {