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/pdf/PdfNoteSheet.jsx

@@ -12,7 +12,7 @@ import { useEffect, useRef, useState } from "react";
  * The page stays visible above it, so the passage the note is about is still
  * there while writing about it.
  */
-export function PdfNoteSheet({ T, note, number, onChange, onClose, onRemove }) {
+export function PdfNoteSheet({ T, note, number, onChange, onClose, onRemove, link }) {
   const [draft, setDraft] = useState(note.text);
   const areaRef = useRef(null);
 
@@ -109,6 +109,55 @@ export function PdfNoteSheet({ T, note, number, onChange, onClose, onRemove }) {
         }}
       />
 
+      {/* Same two ways out as the margin card: some remarks turn into a page of
+          their own, and the pin should lead there afterwards. */}
+      <div style={{ display: "flex", alignItems: "center", gap: 14, marginTop: 8 }}>
+        {note.noteId ? (
+          <button
+            data-pin-goto={note.noteId}
+            onClick={() => link.onOpenNote(note.noteId)}
+            style={{
+              display: "flex",
+              alignItems: "center",
+              gap: 5,
+              flex: 1,
+              minWidth: 0,
+              border: "none",
+              background: "transparent",
+              color: T.accent,
+              cursor: "pointer",
+              fontSize: 12,
+              fontWeight: 600,
+              padding: 0,
+              textAlign: "left",
+            }}
+          >
+            <span style={{ flexShrink: 0 }}>→</span>
+            <span style={{ overflow: "hidden", textOverflow: "ellipsis", whiteSpace: "nowrap" }}>
+              {link.labelOf?.(note.noteId) ?? "Notiz gelöscht"}
+            </span>
+          </button>
+        ) : (
+          <button
+            data-pin-make-note
+            onClick={() => {
+              commit();
+              link.onCreate(draft);
+            }}
+            style={{
+              border: "none",
+              background: "transparent",
+              color: T.accent,
+              cursor: "pointer",
+              fontSize: 12,
+              padding: 0,
+            }}
+          >
+            Notiz anlegen
+          </button>
+        )}
+      </div>
+
       <button
         onClick={close}
         style={{