Add regression tests for the bugs found in this smoke test:
- resolve_workspace: by-name, by-UUID, unknown-identifier, current-fallback,
actionable no-workspace message.
- DateTimePicker: selected-day highlight must be month-scoped; committing
after navigating months uses the selected month, not the viewed one.
- create_task: version is saturating_add on u64::MAX (doesn't panic/wrap).
Also fixes the three pre-existing clippy warnings (WorkspaceMode now uses
#[derive(Default)] + #[default], repository test drops unused binding,
sync test uses struct-update syntax instead of field-reassign-default).
a79dcc4617