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Digit Kanban now connects to the digitata-kanban GraphQL API to fetch
actual boards, columns, and cards instead of using sample data.

Changes:
- Add Qt Network module for HTTP/GraphQL communication
- Create ApiClient class for GraphQL queries
  * Supports boards, columns(boardId), cards(columnId) queries
  * API key authentication via X-API-Key header
  * Async data loading with Qt signals/slots
- Update MainWindow to use API client
  * Load data from API on startup
  * Fallback to sample data if API not configured
  * Cache columns and cards for offline viewing
- Add configuration support
  * Environment variables: KANBAN_API_ENDPOINT, KANBAN_API_KEY
  * QSettings config file support (~/.config/Digitata/DigitKanban.conf)
  * config.ini.example for reference
- Update README with API configuration instructions

GraphQL Schema Integration:
- Query boards: { boards { id name description created_at } }
- Query columns: { columns(boardId: ID!) { id name position cards } }
- Query cards: { cards(columnId: ID!) { id title description status } }

API Authentication:
- X-API-Key header support for authenticated requests
- Configurable via env vars or settings file

Binary Size: 239 KB (Linux) - increased from 150 KB due to Qt6::Network
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>