feat: add copy-link action to copy the page's .md URL

[azizbecha]

Summary

Adds a copy-link dropdown action that copies a link to the current page's Markdown (.md) URL to the clipboard.

Today the dropdown can open the markdown (view → "View as Markdown") or copy the rendered content (copy → "Copy page"), but there's no way to grab a link to the .md file — you have to open it and copy the address bar. This adds that as a one-click action.

It resolves the URL through the existing resolveContextUrl, i.e. the same URL the "Open in ChatGPT/Claude/…" actions reference, so it honours markdownUrl / generateMarkdownRoutes. On a site that serves per-page markdown it copies the .md link; otherwise it falls back to the page URL (the same degradation as the AI actions).

Changes

• New copy-link item in allDropdownItems, backed by a small copyMarkdownLink action (reuses resolveContextUrl + writeTextToClipboard).
• Added 'copy-link' to DEFAULT_ACTIONS.
• ActionId 'copy-link' and labels.copyLink added to react.d.ts and index.d.ts.
• README (available actions, default list, i18n table) and CHANGELOG updated.

Notes / open question

• Default-on: I added it to DEFAULT_ACTIONS, so the item shows for existing consumers on upgrade. If you'd rather avoid a default UX change, I'm happy to make it opt-in (just drop it from DEFAULT_ACTIONS) — your call.
• Labels default to Copy link to Markdown / Copy this page's .md URL, overridable via labels.copyLink and i18n ids copyPageButton.copyLink.*.

Test plan

• npm test passes (existing postBuild route-generation tests).
• Component parses as valid JSX; the new item mirrors the existing view item's structure.
• Manually verified the action copies the resolved markdown URL.