feat: drag-and-drop reorder for Smart Clipboard boards and snippets

[h3qing]

What

Add drag-and-drop reordering to the Smart Clipboard Kanban board:

• Reorder snippets within a board
• Move snippets between boards
• Reorder boards themselves

Current state

The UI already has grip handles (<GripVertical> icons) on each snippet card, and boards are rendered as horizontal columns. The position field exists in both bf_snippets and bf_snippet_boards tables. The update APIs (whisperwoofScUpdateSnippet, whisperwoofUpdateBoard) already support changing position and boardId.

Approach

Use a lightweight drag library like @dnd-kit/core (already in the React ecosystem, ~8KB gzipped):

1. Wrap each BoardColumn in a SortableContext
2. Make SnippetCard draggable via useSortable
3. On drop: update position (and boardId if cross-board) via the existing IPC methods
4. Persist new positions to SQLite

Files to modify

• src/whisperwoof/ui/smart-clipboard/SmartClipboard.tsx — add dnd-kit wrappers
• package.json — add @dnd-kit/core + @dnd-kit/sortable dependencies

Acceptance criteria

• Snippets can be reordered within a board via drag
• Snippets can be moved between boards via drag
• New positions persist across app restart
• Smooth drag animation (no jank)