Add Trello card creation and completion UI (Phase 3 Step 3)

Implement interactive Trello card management with HTMX:

Frontend:
- Create trello-board.html partial with add card form
- Add collapsible form with list selector and card title input
- Add completion checkbox on each card
- Update trello-boards.html to use new partial
- Use HTMX for seamless partial updates (hx-post, hx-swap)

Backend:
- Add HandleCreateCard: creates card and re-renders board
- Add HandleCompleteCard: marks card as closed
- Register /cards and /cards/complete POST routes

Features:
- Add cards to any list via dropdown
- Mark cards complete with checkbox (removes from view)
- Real-time board updates without full page reload
- Glassmorphism styling for form

All tests pass. Full Trello write operations now available in UI.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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