feat(users): migrate UsersDao, UsersRepository and UserManager to coroutines

[mahibi]

Migrates user account handling (UserManager, UsersRepository, UsersDao) from RxJava to Kotlin coroutines, then updates every call site in the app to use the new suspend API.

Commit 1 — data layer (feat(users): migrate UsersDao, UsersRepository and UserManager to coroutines)

• UsersDao: abstract class → interface, every DB method becomes suspend fun; getActiveUserObservable(): Observable → getActiveUserFlow(): Flow<UserEntity?>.
• UsersRepository/UsersRepositoryImpl: mirrors the DAO — Single/Maybe → suspend fun, Observable → Flow.
• UserManager: internals rewritten as suspend functions. Every existing public Rx-returning method is kept as a @deprecated, one-line rxSingle/rxMaybe wrapper around the new suspend function, so
  existing callers keep compiling.
• Tests and the Compose-preview dummy DAO updated to match.

Commit 2 — call sites (feat(users): migrate UserManager call sites off RxJava wrappers)

• Switches every call site (Activities, ViewModels, Compose, Workers, BroadcastReceivers, the OkHttp interceptor) off the deprecated wrappers and onto the suspend functions, using
  lifecycleScope/viewModelScope/rememberCoroutineScope() where a coroutine scope is available, and runBlocking where the call site must stay synchronous (onCreate/onResume, Worker.doWork()/init,
  BroadcastReceiver.onReceive) — a like-for-like swap, since .blockingGet() already blocked the same thread.
• Fixes a few dormant bugs found along the way, where a returned Single/Maybe was never subscribed to, so the update silently never ran: ConversationsListActivity.handleEcoSystemIntent,
  SettingsActivity's client-cert and profile-refresh updates, PushRegistrationWorker.webPushUnregistrationWork.
• Leaves the plain Java Worker classes and KeyManager on the deprecated Rx wrappers (Java can't call Kotlin suspend funs without manual Continuation handling), and leaves UserManager.currentUser call
  sites in DiagnosisElement, LocalLoginDataSource, and CurrentUserProviderOldImpl on Rx, since its "pick any user and mark it active" fallback isn't exposed as a suspend function and the documented
  replacement (CurrentUserProvider.getCurrentUser()) has different semantics.

Recommended follow-ups

• Convert the remaining plain Worker subclasses (NotificationWorker, PushRegistrationWorker, ShareOperationWorker) to CoroutineWorker, matching ReadMarkerSyncWorker/ChatMessageCatchUpWorker, so their suspend calls stop needing runBlocking.
• Use goAsync() + a coroutine scope in the four BroadcastReceivers for the same reason.
• Decide on a path for the Java-only call sites (CapabilitiesWorker, DeleteConversationWorker, SignalingSettingsWorker, WebsocketConnectionsWorker, AccountRemovalWorker,
  AddParticipantsToConversationWorker, KeyManager) — either convert them to Kotlin or accept they'll stay on the deprecated Rx wrappers indefinitely.
• If CurrentUserProvider.getCurrentUser() should eventually replace UserManager.currentUser everywhere, its fallback behavior (auto-picking and activating a user when none is marked active) needs to be
  decided on and ported over first.

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/34246126796/artifacts/10064452703
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
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[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/34466901916/artifacts/10150441248
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/34849556867/artifacts/10350730917
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/35243431480/artifacts/10506924023
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)