[schedule] Reduce render, drag and network overhead

[frankrousseau]

Problem

• Document-level mousemove listeners forced a layout on every mouse move anywhere on the page, and drag processing ran once per event instead of once per frame
• Each task-type expand fetched the tasks and entities twice, refetched the days off every time, and filtered entities with O(entities x tasks) scans
• Applying assignments issued one sequential request per entity for versioned tasks and for task updates
• Render-time helpers reallocated day-off ranges, timesheet filters and selection scans for every bar on every render
• setItemPositions allocated a cell per day of the range on every drag tick and weeksAvailable reparsed a moment per day just to keep the Mondays

Solution

• Attach move listeners only during drags, cache the wrapper rect and throttle drag work with requestAnimationFrame
• Reuse the expand loads for the side panel, replace the linear scans with Sets/Maps and cache the days off per date range
• Load versioned tasks once per assignment run and flush task updates in small parallel batches
• Memoize the day-off ranges and timesheet groups, make isSelected a Set lookup
• Pack timebar lines with per-line interval lists and iterate the week headers Monday to Monday

Note: stacked on #2141, only the 14 perf commits are new here.