Preserve TOC state in shared links and embed URLs

[Copilot]

The TOC view state was not carried through when sharing a pad or copying embed code, so recipients could open the same pad with a different layout. This updates both the live pad URL and the share/embed values so the selected TOC view travels with the link.

• Sync TOC state into shareable URLs

  • Add toc=true|false to the current pad URL when the TOC toggle changes.
  • Rewrite the share link and embed iframe src to preserve the same TOC state.

• Handle Etherpad share dialog regeneration

  • Re-apply the TOC parameter after Etherpad regenerates share/embed fields, including readonly mode changes.

• Regression coverage

  • Add focused tests for:
    • URL parameter rewriting
    • embed src rewriting
    • share field synchronization
    • current-location URL synchronization

• Example

  <iframe
    name="embed_readwrite"
    src="https://example.com/p/test-pad?showControls=true&showChat=true&toc=true"
    width="100%"
    height="600"
    frameborder="0"></iframe>

  Shared pad URLs now follow the same pattern:

  https://example.com/p/test-pad?showChat=false&toc=true