- Rewrote _kanbanRenderMarkdown() from basic paragraph wrapper to a
line-by-line block processor supporting headings, code blocks, lists,
task lists, tables, blockquotes, horizontal rules, and strikethrough.
- Added CSS for all new elements (table borders, code blocks, checkboxes,
blockquote accent, heading sizing, etc.).
- Dropped white-space: pre-wrap from .kanban-task-preview-body and
.kanban-detail-row-main since markdown now handles layout.
- Applied _kanbanRenderMarkdown() to task description (was esc()) and
comment body (was esc()) in the task detail view.