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
M57
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