Pure helpers for the Gantt's drag/double-click math:
- fractionToDate(fraction, range): what ISO date a fractional
position in the chart represents, snapped to whole UTC days.
- shiftDatesByFraction: move a bar's start and target by the
same delta, clamped to the visible range, with start dropped
when clamping would invert or collapse the span.
- resizeEndToFraction: what the new target date becomes when
the user drags a right edge.
- moveCardByFraction: alias that produces the action's input
shape.
11 unit tests cover the corners (no dates set, point bar,
clamping past the right edge, inversion guard).
rescheduleCardAction also drops its owner-only gate. Any
signed-in user — owner or client manager — can move dates
now. PocketBase's existing row-level rules govern which cards
a given user can write (a client manager still only writes
their own client's cards); the activity-log row is unchanged
and attributes the move to the actual user either way.
Adds setSubtaskDatesAction so the same drag/double-click
gestures work on subtask rows. No audit log on subtasks —
matches the existing pattern (subtask toggles don't log).
