gtsync: Sync Tasks & Reminders

New features: • Pick which Apple Reminders account new lists are created in, both during onboarding and later in Settings. • New Siri Shortcuts setup guide with step-by-step screenshots, reachable from Settings under Background Sync. • Cancel a backup restore while it's still in the safe phase. • New app icon. Improvements: • Sync-lock retry button renamed to "Retry Without Taking Over" when a takeover is possible, making the choice explicit. • Daily backups fall back to alternate destinations when the preferred destination is unavailable, instead of silently failing. • Activity-log retention now defaults to on for new users, so the activity log self-prunes by default. • Backups settings warn clearly when daily backups are off, when storage is set to Local only, and when reminders sync via iCloud (where backups can duplicate items on a fresh device). • The Activity log view loads quickly even with very long histories. • Backup imports accept both compressed and plain JSON files. • The backup-action spinner now appears on the button you actually tapped. • Restore overlay shows the current step (Preparing, Safety backup, Restoring, Finalizing), with the Cancel button emphasized and reliably tappable. • The tab bar is hidden while a backup restore is in progress, so the restore can't be interrupted by switching tabs. • Apple Reminders authorization refreshes when returning to onboarding or Settings from System Settings. • The Apple Reminders source picker is hidden when only one source is available, and the wording around the "Automatic" option is clearer. • Google sign-in stays valid longer near token expiry edges. • Google Tasks list collection caches across launches, so list changes are detected faster on the next sync. • Backup creation cleans up its temporary files even when it fails partway. • Apple Reminders source picker stays visible when the previously selected account is temporarily missing, so it can be reselected. • Google sign-in screen now explains why signing in with Google is necessary. Bug fixes: • Restoring a Google Drive backup during onboarding now creates the pre-restore safety backup it was supposed to create. • Background sync no longer reads from the keychain when background sync is disabled. • The activity log retention cutoff no longer drifts away from the user's chosen retention window. • Backup app-version field shows "unknown" instead of a misleading "1.0" when it can't be read from the backup.