Enable preview task checkboxes

[PathGao]

Fixes #319

Comrak emits task checkboxes without the legacy class or data marker, leaving every preview task disabled. This change tags only renderer-confirmed task list items at the Rust boundary, then makes the preview activate only those items.

The toggle now uses the rendered source line rather than DOM checkbox order. It therefore updates the intended unordered, ordered, nested, quoted, and front-matter-bearing task while raw HTML controls and code-fence lookalikes remain inert.

This is stacked after #318 and its predecessors. Merge the predecessor chain first.

Validation

• npm ci and npm audit (0 vulnerabilities)
• npm run check (0 errors, 0 warnings)
• npm test (139 passing)
• npm run build
• cargo test (25 passing)
• Added renderer regressions for unchecked/checked, nested/quoted, raw-HTML, and multiline-wikilink cases.

Post-Deploy Monitoring & Validation

No additional operational monitoring required: rendering stays local and no service, telemetry, or persistent schema changes. Owner: maintainer/reporter; validate by toggling Markdown tasks in preview after merge, including an ordered task and an inert raw HTML checkbox.

Fixes #148.

[PathGao]

@alecdotdev This fixes the current preview task-checkbox regression from #148/#319. It is stacked after #318 and its predecessors; please review in that order.

[PathGao]

Superseded by native Stack PR #345 (Stack #353). Closing this fork-head PR so #345 is the active review path.