fix(web): keep multi-week drag source

[tyler-dane]

Summary

• retain the canonical source event while a drag navigates between week queries
• allow timed and all-day drops to build and issue their saved-event mutation in the destination week
• cover both event types when the visible-week map no longer contains the source

Related to #1784 and #1786.

Simplicity

• keeps the fix inside the Week interaction coordinator and reuses the existing strict mutation pipeline
• avoids new cross-layer state or changes to drag geometry
• no separate simplification commit was needed after removing unnecessary teardown control flow

Manual Testing Steps

• Drag a timed event to the next week, drop it in a different day column, and confirm it appears immediately in that slot
• Drag an all-day event to the previous week, drop it in a different day column, and confirm it appears immediately in that slot
• Confirm both interactions leave the browser console free of errors or warnings

Test plan

• bun test:web -- WeekInteractionCoordinator week-interaction.timed-drag week-interaction.all-day-drag
• bun type-check
• bun lint
• bun run verify
• local browser validation on the Week view for timed next-week and all-day previous-week drops