Mobile Schedule Redesign

The schedule screen now features a fully redesigned mobile view:

• Card-based layout — Shifts and leave display as easy-to-read cards instead of a cramped table
• Swipe navigation — Swipe left/right to move between days
• Quick add shifts — Tap any cell to add a new shift directly
• Group selector — Switch between Location and Staff grouping via the floating action button

Bug Fixes

• Schedule menu item now appears in mobile navigation
• Shift publish errors now show proper error notifications (OPT-78)