Includes changes from prior sessions: Epilogue + Space Mono fonts,
OverlayScrollbars integration, markdown editor fixes, settings dialog,
import/export buttons, and various UI refinements.
Replace 60/40 split layout with a 2x3 CSS grid where all sections
get equal weight. Cover color header, progress bar on checklist,
compact markdown editor, scroll containment on long lists.
- CardDetailModal: two-panel layout (60/40) with inline title editing
- MarkdownEditor: edit/preview toggle with react-markdown + remark-gfm
- ChecklistSection: add/toggle/edit/delete items with progress counter
- LabelPicker: toggle labels + create new labels with color swatches
- DueDatePicker: date input with relative time and overdue styling
- AttachmentSection: list with remove, placeholder add button
- Wired into BoardView via selectedCardId state
