feat(editor)!: reader highlights, re-anchoring, and a page frame that takes presses

[clearloop]

Lands in 0.1.27.

• Reader highlights: markdown::HighlightColor, Annotation::Highlight, set_highlight_paint / default_highlight, and ToggleHighlight (cmd-shift-h) for a registered highlight mark.
• markdown::Quote re-finds an anchor's range after the file changed outside the editor.
• Breaking: editor anchors are no longer comment-only — CommentId → AnchorId, CommentActivated → AnchorActivated, comment_at → anchor_at; Annotation is #[non_exhaustive].
• Relative image paths resolve against a per-document base (Editing::base, Editor::with_base); ImageStore::keep takes that base instead of reading the editor mid-update.
• Editor::press takes presses from a host frame, and drags keep following the pointer past the editor's box.
• Fixes: drop line left behind after a file drag; paste in source mode or a code block goes in as text at the caret.