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)