Let users force a full cache purge of the published site from the
publish popover once the site is live. Guarded by a confirmation
dialog explaining it is only useful for large sites and that pages
are slower on first visit until cached again.

MCP-authored [text](url) links were stored with a non-canonical mark
shape ({ href, linkType }) that the renderer's LinkSettings resolver
ignored, so the href was dropped on published/previewed pages.

- Emit canonical url variable attrs from the MCP inline parser
- Tolerate the legacy { href, linkType } shape in getLinkSettingsFromMark
  so already-authored links resolve at render time
- Normalise attrs via getLinkSettingsFromMark on the React render path
- Fix the sample template link to use the canonical shape