Harden link checks and keep a new link on the text it was opened for

- Refuse control, format and separator characters in link and image URLs,
  in the editor and in the dialect validator.
- Refuse protocol-relative and backslash values as repository paths, in
  both places. This closes a gap the server validator already had.
- Capture the link editor's target as Yjs relative positions. A remote
  edit no longer moves a new link onto other text; if the text itself is
  replaced, nothing is linked and the editor says why.
- Mod+Enter opens the link under the caret; Tab cycles inside the editor
  popover; host:port values become https URLs.
- Start-align both popovers to the link's left edge and follow remote
  edits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>