Remove deprecated sync mode UI and manual sync controls from
SettingsScreen and TasksScreen. Implement an always-full sync mode and a
new auto-sync lifecycle in the app store: startAutoSync/stopAutoSync,
periodic polling (60s), debounced file-change-triggered sync (5s), and
window-focus-triggered sync when last sync is older than 30s. Track
syncStatus (idle/synced/error/offline), lastSyncTime, and surface status
in the Tasks drawer with a status dot and a manual sync button. Wire
auto-sync startup/teardown into loadConfig, switchWorkspace,
addWebdavWorkspace, and removeWorkspace.
These changes simplify sync configuration by removing legacy sync modes
and UI elements, provide robust automatic syncing for WebDAV workspaces,
and improve user feedback on sync state and errors.
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