Link the margin notes from the notes, and let them be hidden

The linking went the wrong way round. A remark in the margin is short by design;
when there is more to say about that passage it belongs in the notes beside the
document — and it has to be possible to say *which* passage. So the pins are
offered to the editor's `@` menu exactly like notes and notebooks are, labelled
with their page, and following one scrolls the document to the spot and opens the
remark. Pins are unhidden first: a link leading to something invisible would read
as broken.

A pin link is the same node with kind "pin", and is deliberately kept out of the
links reported to the server. Its target is a marker inside this note's own
overlay, so no note has that id, and reporting it would fill the backlink table
with rows matching nothing.

They can also be hidden altogether. Reading a document as it was printed is a
normal thing to want; the toolbar takes the markers and the column away together
and the page takes the width back.

The other direction — making a note out of a remark — is removed, along with its
picker and the pin's noteId: 357 lines fewer than before. The unmount flush it
turned up stays, since a drawing save was genuinely being dropped by navigating
away inside its debounce.

218 unit tests, 231 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, link }) {
+export function PdfNoteSheet({ T, note, number, onChange, onClose, onRemove }) {
   const [draft, setDraft] = useState(note.text);
   const areaRef = useRef(null);
 
@@ -109,55 +109,6 @@ export function PdfNoteSheet({ T, note, number, onChange, onClose, onRemove, lin
         }}
       />
 
-      {/* 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={{