Build fully custom CalendarPopover with date-fns + Radix Popover.
Month/year dropdown selectors, today button, clear button, past
dates dimmed. Rewrite DueDatePicker to use it instead of <input type=date>.
- 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
