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/MainActivity.kt

@@ -1,5 +1,6 @@
 package com.m57.hermescontrol
 
+import android.content.Context
 import android.content.Intent
 import android.os.Bundle
 import androidx.activity.ComponentActivity
@@ -16,11 +17,24 @@ import androidx.compose.ui.Modifier
 import com.m57.hermescontrol.data.local.AuthManager
 import com.m57.hermescontrol.notification.NotificationReplyReceiver
 import com.m57.hermescontrol.theme.HermesControlTheme
+import com.m57.hermescontrol.util.LocaleContextWrapper
 
 class MainActivity : ComponentActivity() {
     // Observable state so both onCreate and onNewIntent updates flow to Compose
     private var notificationSessionId by mutableStateOf<String?>(null)
 
+    /**
+     * Apply the user-selected display language before any view is inflated.
+     * Reads the persisted code from [AuthManager]; an uninitialized store
+     * (shouldn't happen here, but guarded) falls back to the device locale.
+     */
+    override fun attachBaseContext(base: Context) {
+        val code =
+            runCatching { AuthManager.getAppLanguage() }
+                .getOrDefault(LocaleContextWrapper.SYSTEM_LANGUAGE)
+        super.attachBaseContext(LocaleContextWrapper.wrapWithCode(base, code))
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)