feat: handle app lifecycle events (Pause/Resume/Background/Foreground)

[TigerInYourDream]

Summary

Implements app lifecycle event handling as described in #458. The app now properly saves/restores state and stops/starts the Matrix sync service when the OS suspends, backgrounds, or resumes the application.

This is especially important on Android, where the OS may pause or kill the app at any time. Previously, only Shutdown was handled — meaning state could be lost if the process was killed without a clean shutdown.

Changes

Single file modified: src/app.rs (+118 / -27 lines)

New lifecycle event handling

Replaced the monolithic if let Event::Shutdown block with a match on lifecycle events:

Event	Action
Pause / Background	Save window geometry + AppState, stop sync service
Foreground / Resume	Restore state, restart sync service, full UI redraw
Shutdown	Save everything including TSP wallet state (destructive)

New methods on App

• save_lifecycle_state — Saves window geometry and AppState to persistent storage. Best-effort: continues saving remaining parts even if one fails.
• save_all_state — Calls save_lifecycle_state + saves TSP wallet state. TSP save is destructive (close_and_serialize consumes TspState), so this is only called on Shutdown.
• restore_all_state — Restores window geometry and AppState. Preserves the current logged_in flag to avoid overwriting it with a stale persisted value.

Deduplication via state_saved flag

A state_saved: bool field on App prevents redundant saves during the outgoing lifecycle sequence (Pause → Background → Shutdown) and gates restores so they only run after a save actually occurred.

Design Decisions

1. TSP only on Shutdown: close_and_serialize() consumes ownership and tsp_init() uses OnceLock (one-time init), so TSP state cannot be round-tripped across Pause/Resume cycles.

2. Idempotent sync service calls: SyncService::stop() and start() are idempotent, so calling them on both Pause+Background (or Resume+Foreground) is safe.

3. Timeouts on all blocking async: All block_on_async_with_timeout calls use 2–3 second timeouts to stay well under Android's 5-second ANR limit.

4. No platform-specific code: All lifecycle events are platform-agnostic Makepad Event variants — no #[cfg(target_os)] needed.

5. Preserve logged_in on restore: The persisted AppState may contain a stale logged_in value, so we explicitly preserve the current runtime value after restoring.

Closes #458