fix(css): align native Markdown task checkboxes

[hakula139]

Summary

• Use native checkboxes as the marker for Markdown tasks in articles and comments, including ordered lists. One shared rule removes the list marker and aligns the checkbox with the first line so wrapped text stays aligned.
• Reuse the theme accent and native light / dark appearance. Support tight and paragraph-wrapped task items, and rebuild the committed stylesheet.

Verification

• pnpm build, pnpm format, pnpm lint, pnpm spellcheck, nix flake check, and git diff --check passed on aarch64-darwin.
• Managed headless Chromium, Firefox, and WebKit checked the REALMS article HTML with compiled CSS in light and dark mode at 320px, 390px, and 1440px. Native checkbox appearance and disabled state remain intact, task items use one marker, and the page has no horizontal overflow.
• Mixed, nested, paragraph-wrapped, completed, and ordered task probes passed. Comment task-list fixtures passed with the pinned Twikoo 1.7.20 script and its injected CSS in all three engines. Checked state remains intact in print media. Desktop and mobile screenshots were inspected.
• Browser checks used HTML / CSS overlays and comment fixtures with external resources excluded. Twikoo was not connected to its backend, and no comments were submitted. Native checkbox appearance varies by browser, and disabled controls retain native muted colors. Production deployment and native desktop browsers were not tested.