[Task] MarkdownEditor section + /write page

[soroushm]

Task Description ✏️

The default composition that turns the headless parts into a full editor (section/MarkdownEditor) and the page that hosts it (/write). Subtask of #267.

Acceptance Criteria ✅

• Composes Control + Toolbar + Editor + Preview and owns the Edit · Split · Preview switch (the theme layer ships no switcher).
• Forwards the editor pass-through props and the preview slotProps.
• /write page hosts it, noindex,nofollow, unlisted (not in nav), excluded from the sitemap.
• 100% unit coverage on the section; e2e for /write; README + Storybook story.

Definition of Done 🏁

Lint, typecheck, coverage, e2e, Storybook, build green.