feat(#232): dynamic light/dark theme switch without restart

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

@@ -7,6 +7,8 @@ import androidx.activity.enableEdgeToEdge
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Surface
+import androidx.compose.runtime.collectAsState
+import androidx.compose.runtime.getValue
 import androidx.compose.ui.Modifier
 import com.m57.hermescontrol.data.local.AuthManager
 import com.m57.hermescontrol.notification.NotificationReplyReceiver
@@ -21,11 +23,8 @@ class MainActivity : ComponentActivity() {
 
         enableEdgeToEdge()
         setContent {
-            // B6 (Jun 18 2026, kanban t_86e9be9b): read the stored theme
-            // preference at startup so HermesControlTheme actually applies
-            // the user's SYSTEM/LIGHT/DARK choice rather than defaulting to
-            // SYSTEM on every cold start.
-            HermesControlTheme(themePreference = AuthManager.getThemePreference()) {
+            val themePreference by AuthManager.themePreferenceFlow.collectAsState()
+            HermesControlTheme(themePreference = themePreference) {
                 Surface(
                     modifier = Modifier.fillMaxSize(),
                     color = MaterialTheme.colorScheme.background,

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

@@ -41,6 +41,9 @@ object AuthManager {
     private val _bottomNavItemsFlow = MutableStateFlow<List<String>>(emptyList())
     val bottomNavItemsFlow: StateFlow<List<String>> = _bottomNavItemsFlow.asStateFlow()
 
+    private val _themePreferenceFlow = MutableStateFlow<ThemePreference>(ThemePreference.SYSTEM)
+    val themePreferenceFlow: StateFlow<ThemePreference> = _themePreferenceFlow.asStateFlow()
+
     private val gson = com.google.gson.Gson()
 
     /**
@@ -67,6 +70,7 @@ object AuthManager {
                 )
 
             _bottomNavItemsFlow.value = getBottomNavItems()
+            _themePreferenceFlow.value = getThemePreference()
         }
     }
 
@@ -206,6 +210,7 @@ object AuthManager {
 
     fun setThemePreference(theme: ThemePreference) {
         requirePrefs().edit().putString(KEY_THEME_PREFERENCE, theme.name).apply()
+        _themePreferenceFlow.value = theme
     }
 
     /** Convenience: build the base URL from current host + port.

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

@@ -119,6 +119,7 @@ class SettingsViewModel : ViewModel() {
 
     fun onThemeChange(theme: ThemePreference) {
         _uiState.update { it.copy(themePreference = theme, isSaved = false) }
+        AuthManager.setThemePreference(theme)
     }
 
     fun onTypingEffectEnabledChange(enabled: Boolean) {