MI-T2: Markdown export (page → .md) + remove export gating

[coderhd]

Parent

#26 — H0 — Markdown Import/Export (full round-trip)

What to build

A single page downloads as a clean .md file — YAML frontmatter (title, properties, tags) plus the GFM body serialized by the round-trip engine — reachable from the editor's Export menu. Alongside it, the PRO plan gate on ALL exports is removed: markdown, DOCX, and PDF become free on every tier (no upgrade modal, no PRO badge).

Acceptance criteria

• Editor Export menu gains Download as Markdown (.md); the file downloads as <slugified-title>.md.
• Exported file starts with YAML frontmatter: title, page properties, and tags (from page_tags, falling back to properties.tags), followed by the serialized body.
• DOCX/PDF remain available but the plan gate, upgrade modal, and PRO badge on the Export button are removed — all exports free on every tier.
• Export is unavailable to viewers (menu already renders only for non-viewers).
• Focused unit tests pass (assembled file shape, frontmatter, filename slugification); npm run lint and npm run build green.

Blocked by

• MI-T1: Markdown round-trip engine (serialize/parse + frontmatter) #56 — MI-T1: Markdown round-trip engine (serialize/parse + frontmatter)