Replace the native link prompt with an inline link popover

Adding or editing a link used window.prompt and window.alert. A link
editor now opens under the selection with the current URL prefilled,
inline validation, and Remove/Save actions. A caret on a link (or a
click, for readers) shows a small preview with Open and Edit. Bare
domains and addresses gain https:// or mailto:.

Escape with the selection toolbar showing now collapses the selection
and keeps focus in the editor instead of leaving the toolbar orphaned.

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