MUL-2926 feat(editor): support markdown checkbox task lists (#3593) #3657

Summary

Closes #3593 (MUL-2926). Renders - [ ] / - [x] as interactive checkboxes in the issue content editor, the way GitHub / Notion do.

Before, the read-only view (ReadonlyContent, react-markdown + remark-gfm) already rendered task-list checkboxes, but the editable Tiptap editor parsed - [ ] as a plain bullet with literal [ ] text. This PR brings the editable view to parity.

Changes

• Register TaskList + a patched TaskItem in the shared extension factory (extensions/index.ts). Both ship their own markdown tokenizer / renderMarkdown, an input rule (typing [] / [x] ), and a checkbox NodeView. The taskList tokenizer is consulted before marked's built-in list tokenizer, so - [ ] becomes a task while a plain - still falls through to the existing bullet list — no conflict.
• Patch TaskItem's keymap to reuse PatchedListItem's split → lift Enter chain, so double-Enter on an empty task item exits the list (same fix bullet lists already have). nested: true enables sub-tasks and nested markdown round-trips. The shared chain is factored into a small listItemKeymap helper.
• Bubble menu: add a "Task list" entry to the list dropdown, with i18n strings for en / zh-Hans / ja / ko.
• Styling (styles/prose.css): task lists are styled for both the editor (ul[data-type="taskList"]) and the read-only remark-gfm output (ul.contains-task-list), so both views match. Checkboxes use accent-color: var(--brand); completed items render muted.

Testing

• pnpm --filter @multica/views run typecheck — clean
• eslint on changed files — clean
• pnpm --filter @multica/views test — 430 passed, including:
  • New editor/extensions/task-list-markdown.test.ts: round-trips - [ ] / - [x] (parse + serialize) through a real editor, confirms uppercase [X], confirms a plain bullet stays a bullet, and confirms a checkbox toggled in the editor serializes back to - [x].
  • New readonly case: - [x] renders a checked, disabled checkbox (checked state preserved).
  • Locale parity test passes with the new key.

Verified the produced DOM in jsdom for both modes to confirm the CSS selectors match (the editor's checkbox NodeView keeps data-checked on the <li>; readonly emits .task-list-item with the checked attribute).

🎬 Cutter preview — PR #3657

Toggle a task list item checkbox — Markdown checkbox task lists render in the editor, with checked items shown muted.