Let a margin remark become a note, and lead to it ever after

Some remarks turn into a page of their own. "Notiz anlegen" on a margin card
makes one, titles it from what was written, and goes there; the pin keeps its id
so the passage leads to that note afterwards. The note is created with a link
back to the document, so the connection reads both ways and the document lists it
among its backlinks — a note born from a passage should not be a note with
nothing on it saying which passage. An existing note can be linked instead, and a
link can be let go of without touching the note. Both are offered on the phone
sheet too.

Two older bugs stood in the way and are fixed:

`openLinkTarget` finds a note's notebook by looking it up in the notes list, and
a note created in the same tick is not in the copy that callback closed over — so
jumping to a fresh note went nowhere at all. Creating one now navigates by the
notebook it already knows.

A drawing save waits 700ms for the next stroke, and leaving the document inside
that window dropped it. That is precisely what turning a remark into a note does,
so the link would have been lost on the way out. The viewer flushes a pending
write when it unmounts, which also covers drawing and navigating away quickly.

215 unit tests, 229 browser checks across eleven suites.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/features/notes/EditorPane.jsx

@@ -230,6 +230,8 @@ export function EditorPane({
             onOpenNote={linkProps.openLinkTarget}
             searchLinkTargets={linkProps.searchLinkTargets}
             resolveLinkLabel={linkProps.resolveLinkLabel}
+            onCreateNoteForPin={linkProps.createNoteForPin}
+            onOpenCreatedNote={linkProps.openCreatedNote}
           />
         ) : (
           <NoteEditor