feat(auth): handle server-side deletion of inactive sessions

[MaxDac]

Description

Handles the server-side cleanup from MaxDac/Privee#178. The server now deletes inactive sessions, then answers their log in with 401 invalid_credentials. The API is unchanged and there is no api_version bump.

• Clear message: the new KnownSessionStore records, encrypted under noBackupFilesDir, the session names and ids signed into on each server (sessions.bin). If a log in to one of those names gets a 401 or 404, the auth screen says the session no longer exists on the server because it was deleted after inactivity. It also shows a Create a new session button that opens registration with the name already filled in. The message is translated into en, it, pt-PT, es and fr. Unknown names still get the generic "wrong credentials" message.
• Push re-registration: the new PushEndpointSync keeps the endpoint returned by the distributor and sends it again with PUT /api/app/push after every new sign-in. The existing register-on-sign-in flow is unchanged. The cached endpoint is cleared on sign-out and when the distributor unregisters the app.
• Stale state (optional item): if a known name signs in again with a different id, the old session was deleted. The app then deletes that id's signal-<id>.bin. "Forget device" also removes the name from the store.
• Docs: ARCHITECTURE.md and TECHNOLOGIES.md.

E2EE is unaffected. No keys or content are added to logs, push payloads or backups.

Closes #32

Checklist

• No secrets committed (API keys, endpoints, tokens, .jks, .env)
• :core:* modules contain no android.* / androidx.* imports
• Tests pass (./gradlew test)
• Commit messages follow Conventional Commits

Validation: ./gradlew test :app:lintDebug :app:assembleDebug passed locally. New tests: KnownSessionStoreTest, PushEndpointSyncTest, AuthProblemTest, and an invalid_credentials log-in case in PriveeApiTest. Not done: an end-to-end run on an emulator against a local server.