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/ui/settings/SettingsScreen.kt

@@ -163,6 +163,8 @@ fun SettingsScreen(
                             onUseDynamicColorsChange = viewModel::onUseDynamicColorsChange,
                             themePreset = state.themePreset,
                             onThemePresetChange = viewModel::onThemePresetChange,
+                            appLanguage = state.appLanguage,
+                            onAppLanguageChange = viewModel::onAppLanguageChange,
                         )
 
                         ChatSection(

app/src/main/java/com/m57/hermescontrol/ui/settings/SettingsViewModel.kt

@@ -39,6 +39,7 @@ data class SettingsUiState(
     val selectedProfileId: String? = null,
     val renameProfileName: String = "",
     val bottomNavDisplayMode: BottomNavDisplayMode = BottomNavDisplayMode.ICON_AND_TEXT,
+    val appLanguage: String = "system",
     // Profile add/edit dialog
     val showProfileDialog: Boolean = false,
     val editingProfileId: String? = null,
@@ -79,6 +80,7 @@ class SettingsViewModel(
         val typingEffectDelayMs = AuthManager.getTypingEffectDelayMs()
         val profiles = AuthManager.getConnectionProfiles()
         val bottomNavDisplayMode = AuthManager.getBottomNavDisplayMode()
+        val appLanguage = AuthManager.getAppLanguage()
         val renameProfileName =
             profiles.firstOrNull { p -> p.id == selectedId }?.name ?: ""
         _uiState.update {
@@ -97,6 +99,7 @@ class SettingsViewModel(
                 selectedProfileId = selectedId,
                 renameProfileName = renameProfileName,
                 bottomNavDisplayMode = bottomNavDisplayMode,
+                appLanguage = appLanguage,
             )
         }
     }
@@ -296,6 +299,11 @@ class SettingsViewModel(
         AuthManager.setThemePreset(preset)
     }
 
+    fun onAppLanguageChange(code: String) {
+        _uiState.update { it.copy(appLanguage = code, isSaved = false) }
+        AuthManager.setAppLanguage(code)
+    }
+
     fun onBottomNavDisplayModeChange(mode: BottomNavDisplayMode) {
         _uiState.update { it.copy(bottomNavDisplayMode = mode, isSaved = false) }
         AuthManager.setBottomNavDisplayMode(mode)

app/src/main/java/com/m57/hermescontrol/ui/settings/components/AppearanceSection.kt

@@ -1,5 +1,6 @@
 package com.m57.hermescontrol.ui.settings.components
 
+import androidx.activity.compose.LocalActivity
 import androidx.compose.animation.AnimatedVisibility
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
@@ -43,6 +44,7 @@ import com.m57.hermescontrol.theme.BottomNavDisplayMode
 import com.m57.hermescontrol.theme.ThemePreference
 import com.m57.hermescontrol.theme.ThemePreset
 import com.m57.hermescontrol.ui.settings.SectionCard
+import com.m57.hermescontrol.util.LocaleContextWrapper
 
 @Composable
 internal fun AppearanceSection(
@@ -52,6 +54,8 @@ internal fun AppearanceSection(
     onUseDynamicColorsChange: (Boolean) -> Unit,
     themePreset: ThemePreset,
     onThemePresetChange: (ThemePreset) -> Unit,
+    appLanguage: String,
+    onAppLanguageChange: (String) -> Unit,
 ) {
     SectionCard(title = stringResource(R.string.settings_sec_appearance)) {
         Text(
@@ -193,6 +197,42 @@ internal fun AppearanceSection(
                 }
             }
         }
+
+        Spacer(modifier = Modifier.height(16.dp))
+
+        Text(
+            text = stringResource(R.string.settings_item_language),
+            style = MaterialTheme.typography.bodyLarge,
+        )
+        Spacer(modifier = Modifier.height(8.dp))
+        val languageOptions =
+            listOf(
+                LocaleContextWrapper.SYSTEM_LANGUAGE to stringResource(R.string.language_system),
+                "en" to stringResource(R.string.language_english),
+                "ko" to stringResource(R.string.language_korean),
+            )
+        SingleChoiceSegmentedButtonRow(modifier = Modifier.fillMaxWidth()) {
+            val activity = LocalActivity.current
+            languageOptions.forEachIndexed { index, (code, label) ->
+                SegmentedButton(
+                    selected = appLanguage == code,
+                    onClick = {
+                        onAppLanguageChange(code)
+                        // MainActivity is a plain ComponentActivity (not
+                        // AppCompatActivity), so the locale only takes effect
+                        // after the activity is recreated.
+                        activity?.recreate()
+                    },
+                    shape =
+                        SegmentedButtonDefaults.itemShape(
+                            index = index,
+                            count = languageOptions.size,
+                        ),
+                ) {
+                    Text(label)
+                }
+            }
+        }
     }
 }