feat(sync): wire TUI account orchestration, SyncProvider integration, and Ctrl+S keybinding

- MicrosoftAccountSyncProvider: implements SyncProvider interface with
  connect_account(), sync_account(), disconnect_account(), provider_id()
- TuiApp: inherits SyncObserver, implements thread-safe sync progress
  rendering, background sync thread, and Action::sync dispatch
- CLI: add "account disconnect <id>" and "account list" commands
- Main wiring: initialize sync infrastructure when connected accounts
  exist and a valid client ID is configured
- Screen: add sync_stage and sync_is_error fields to ScreenState,
  map Ctrl+S to Action::sync keybinding
- Fix ScreenState aggregate initializers in all construction sites
- Tests: new microsoft_sync_provider_tests.cpp, all 22 tests pass

tests/tui_tests.cpp

@@ -118,6 +118,8 @@ void test_full_month_render()
         .search_result_index = 0,
         .notification = {},
         .notification_is_error = false,
+        .sync_stage = {},
+        .sync_is_error = false,
         .show_calendar_picker = false,
         .calendar_index = 0,
         .show_agenda = false,
@@ -178,6 +180,8 @@ void test_compact_and_input()
         .search_result_index = 0,
         .notification = {},
         .notification_is_error = false,
+        .sync_stage = {},
+        .sync_is_error = false,
         .show_calendar_picker = false,
         .calendar_index = 0,
         .show_agenda = false,