Add WebDAV configuration, credential storage/testing, and sync controls
across Flutter UI and Rust API. This implements a stateful Settings
screen with fields to enter server URL, username, and password, plus
Test and Save actions; persist/load credentials and workspace WebDAV URL
via the Rust API; add sync mode selection, a Sync Now action, and a sync
status indicator in Tasks screen; thread has_time through date/time
pickers, new task creation, task detail updates, and task DTOs;
implement async Rust functions for testing connections, storing/loading
credentials, setting workspace WebDAV config, and triggering workspace
sync with a SyncResult mapped back to Flutter; add tokio runtime
dependency. These changes were needed to enable full WebDAV-based
synchronization and provide users controls and feedback for configuring
and running syncs from the Flutter app.
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