Add done items feature for Later list

[gusaiani]

Summary

• Later items can be marked as done via a green ✓ button (to the left of the red ✕)
• "See all Done" link below the Later list navigates to /done-list
• Done page shows stats (avg done/week over 10 weeks, this week, this month) and an infinite-scroll list of completed items (50 per page)
• Backend: done_items table, POST /done, GET /done, GET /done/stats endpoints
• Works for both signed-in users (server) and guests (localStorage)

Test plan

• Mark a later item as done → it disappears from the list
• Click "See all Done" → navigates to /done-list with the item listed
• Stats show correct counts
• Infinite scroll loads more items after 50
• Guest mode stores done items in localStorage
• pytest passes (45 tests including 9 new)