Task List UI: Add bulk operations and improve checkbox UX

[omi-discord-vector]

User l4w1 reported in Discord #task-list-ui (today):

• No bulk delete for tasks
• Checkboxes too small to tap
• No "clear completed" button

Suggested fixes:

• Bulk select/delete
• Larger checkbox tap targets
• Clear completed button
• Swipe gestures for mobile

cc @mohsinmsultan @AravGarg