Fix main-thread decryption block by caching token in StateFlow (#322)

Issue #278 reported that `AuthManager.getToken()` (which reads from
`EncryptedSharedPreferences`) was being called synchronously inside the
`MainNavigation` composable, causing UI thread blocks during navigation
transitions (PERF-10).

This commit resolves the issue by caching the token in a new `tokenFlow`
(`MutableStateFlow`) within `AuthManager`.
- The flow is initialised with the token during `AuthManager.init()`.
- Updates to the token (`setToken`, `setSelectedProfileId`,
  `setProfileToken`) are synchronised with `tokenFlow`.
- `MainNavigation` now correctly observes this state using
  `collectAsState()`, preventing disk-read blocking on recompositions.

Impact: Navigation transitions are now fully non-blocking. No side effects
are expected as state synchronisation maintains parity with persistent storage.

Resolves #278

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

@@ -326,7 +326,8 @@ private fun appEntryProvider(
 
 @Composable
 fun MainNavigation(sessionId: String? = null) {
-    val hasToken = !AuthManager.getToken().isNullOrBlank()
+    val token by AuthManager.tokenFlow.collectAsState()
+    val hasToken = !token.isNullOrBlank()
     val startScreen: NavKey = if (hasToken) ChatScreen else ConnectScreen
 
     val backStack = rememberNavBackStack(startScreen)

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

@@ -57,6 +57,10 @@ object AuthManager {
 
     private val _bottomNavDisplayModeFlow = MutableStateFlow<BottomNavDisplayMode>(BottomNavDisplayMode.ICON_AND_TEXT)
     val bottomNavDisplayModeFlow: StateFlow<BottomNavDisplayMode> = _bottomNavDisplayModeFlow.asStateFlow()
+
+    private val _tokenFlow = MutableStateFlow<String?>(null)
+    val tokenFlow: StateFlow<String?> = _tokenFlow.asStateFlow()
+
     private val gson = com.google.gson.Gson()
 
     /**
@@ -84,6 +88,7 @@ object AuthManager {
             _useDynamicColorsFlow.value = isUseDynamicColors()
             _themePresetFlow.value = getThemePreset()
             _bottomNavDisplayModeFlow.value = getBottomNavDisplayMode()
+            _tokenFlow.value = getToken()
         }
     }
 
@@ -122,6 +127,9 @@ object AuthManager {
         token: String?,
     ) {
         requirePrefs().edit().putString("token_$profileId", token).apply()
+        if (getSelectedProfileId() == profileId) {
+            _tokenFlow.value = token
+        }
     }
 
     fun getSelectedProfileId(): String? {
@@ -131,6 +139,7 @@ object AuthManager {
 
     fun setSelectedProfileId(id: String?) {
         requirePrefs().edit().putString(KEY_SELECTED_PROFILE_ID, id).apply()
+        _tokenFlow.value = getToken()
     }
 
     // ── Token ────────────────────────────────────────────────────────────
@@ -150,6 +159,7 @@ object AuthManager {
             setProfileToken(selectedId, token)
         } else {
             requirePrefs().edit().putString(KEY_TOKEN, token).apply()
+            _tokenFlow.value = token
         }
     }