Add Markdown write and preview modes to task forms #84

Work

Use the shared renderer for new task and task detail descriptions. Provide a compact Write/Preview switch and concise syntax help. Keep unsaved content, focus, current version, assignment and navigation state across switches and external updates. Render compact excerpts on cards without nested anchors.

Acceptance

• Multiline lists, fenced code, tables and ordinary descriptions remain usable on narrow screens.
• Preview never saves or changes task status.
• Existing full-card navigation and normal-weight subtask titles remain accessible.
• Meaningful UI behavior is verified, especially dirty form plus external update.

Dependencies

Markdown renderer under #74.