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/App.jsx

@@ -34,6 +34,7 @@ import { EditorPane } from "./features/notes/EditorPane.jsx";
 import { HistoryModal } from "./features/notes/HistoryModal.jsx";
 import { DrawingLayer } from "./features/drawing/DrawingLayer.jsx";
 import { CANON_W, CANON_H, PREVIEW_DRAWING_W } from "./features/drawing/strokes.js";
+import { serializeNoteContent } from "./editor/content";
 
 /** Views that are not a notebook and therefore cannot hold a new note. */
 const VIRTUAL_FOLDERS = ["all", "starred", "trash", "dashboard"];
@@ -279,9 +280,70 @@ export default function App() {
     [notes, openFolder, openNote]
   );
 
+  /**
+   * Turns a remark in a PDF's margin into a note of its own.
+   *
+   * The new note opens with a link back to the document it came from, so the
+   * connection reads in both directions: the pin leads here, and this note shows
+   * up in the document's backlinks. Without that a note born from a passage
+   * would have nothing on it saying which passage.
+   */
+  const createNoteForPin = useCallback(
+    async (source, title) => {
+      const folderId = source.folderId || notebooks[0]?.id;
+      if (!folderId) return null;
+      const note = await data.createNote(folderId);
+      const doc = {
+        type: "doc",
+        content: [
+          {
+            type: "paragraph",
+            content: [
+              { type: "text", text: "Zu " },
+              {
+                type: "noteLink",
+                attrs: { noteId: source.id, label: source.title || "Dokument", kind: "note" },
+              },
+            ],
+          },
+          { type: "paragraph" },
+        ],
+      };
+      data.updateNote(note.id, {
+        title,
+        content: serializeNoteContent(doc),
+        links: [source.id],
+        attachments: [],
+      });
+      return { ...note, title };
+    },
+    [data, notebooks]
+  );
+
+  /**
+   * Opens a note that was created a moment ago.
+   *
+   * `openLinkTarget` looks the target up in the notes list to find its notebook,
+   * and a note created in the same tick is not in the copy that callback closed
+   * over — so it went nowhere at all. Here the notebook is already known.
+   */
+  const openCreatedNote = useCallback(
+    (id, folderId) => {
+      if (folderId) setActiveFolder(folderId);
+      openNote(id);
+    },
+    [openNote]
+  );
+
   const linkProps = useMemo(
-    () => ({ openLinkTarget, searchLinkTargets, resolveLinkLabel }),
-    [openLinkTarget, searchLinkTargets, resolveLinkLabel]
+    () => ({
+      openLinkTarget,
+      searchLinkTargets,
+      resolveLinkLabel,
+      createNoteForPin,
+      openCreatedNote,
+    }),
+    [openLinkTarget, searchLinkTargets, resolveLinkLabel, createNoteForPin, openCreatedNote]
   );
 
   /* ── Actions ── */