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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+# Phase 3 Step 3: Trello UI & Handlers
+
+**Status:** Open
+**Priority:** High
+**Created:** 2024-05-22
+
+## Description
+Implement the UI and backend handlers to enable creating and completing Trello cards directly from the dashboard.
+
+## Requirements
+
+### 1. UI Updates
+*   **Refactor:** Extract individual board rendering into a new partial `web/templates/partials/trello-board.html`.
+*   **Add Card:** Add a "Quick Add" form to each board (using `<details>` for simplicity) that allows selecting a List and entering a Name.
+*   **Complete Card:** Add a checkbox to each card that marks it as complete (archives/closes it).
+
+### 2. Backend Handlers
+*   `HandleCreateCard`:
+    *   POST `/cards`
+    *   Params: `board_id`, `list_id`, `name`
+    *   Action: Call `CreateCard` API.
+    *   Response: Re-render the specific board partial with updated data.
+*   `HandleCompleteCard`:
+    *   POST `/cards/complete`
+    *   Params: `card_id`
+    *   Action: Call `UpdateCard` API (set `closed=true`).
+    *   Response: Empty string (removes the card from UI via HTMX).
+
+### 3. Routing
+*   Register the new routes in `cmd/dashboard/main.go`.
+
+## Implementation Plan
+1.  Create `web/templates/partials/trello-board.html`.
+2.  Update `web/templates/partials/trello-boards.html`.
+3.  Implement handlers in `internal/handlers/handlers.go`.
+4.  Register routes in `cmd/dashboard/main.go`.