fix: close note-switching race conditions and restore offline-first note open

Three issues found in review, all around switching to a different note
while the editor is already open (via a note-link or backlink chip):

- TiptapEditor's onTransaction/onUpdate had no guard against writing
  state after the component is torn down. Removing a focused editor's
  DOM node fires a native blur, which ProseMirror turns into one last
  transaction synchronously during that same teardown pass, crashing
  with state_unsafe_mutation. Deferred the onTransaction write to a
  microtask (checked against a destroyed flag) so it lands after
  Svelte's current pass finishes; guarded onUpdate synchronously.

- openEditor/openNoteById in NotesView.svelte awaited a network fetch
  before ever showing the editor, for every note open (not just
  link-driven navigation) — a regression for an offline-first app, and
  the source of a real race: two rapid note-link clicks left two
  fetches in flight with no ordering guarantee, so whichever resolved
  last won, not whichever was clicked last. Reverted to opening
  instantly from local/IndexedDB data (with a request-id guard against
  the residual race), and instead fetch backlinks lazily in
  NoteEditor.svelte only if not already present — mirroring the
  existing attachments-fetch pattern already in that file. Also
  applied the same post-teardown guard to that pre-existing attachments
  effect, since remounting on note-switch newly exposes it to the same
  class of race.

- FormattingToolbar's closeDropdowns() cleared search results but never
  cancelled the pending debounce timer, so a stale query could
  repopulate results after the dropdown had already closed. Added the
  missing clearTimeout, plus a try/catch around the search fetch for
  consistency with the pattern used elsewhere in the app.

Verified: pnpm check (0 errors), full unit suite (316/316), and the
existing e2e suite including all note-linking scenarios (19/19) all
pass against these changes.

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

src/lib/components/NotesView.svelte

@@ -32,18 +32,8 @@
 	// Use SvelteKit's replaceState (not the raw history API) so the router keeps
 	// ownership of the entry — NoteEditor's close-on-back relies on popstate being
 	// handled by SvelteKit.
-	async function openEditor(note: Note) {
-		try {
-			const res = await fetch(`/api/notes/${note.id}`);
-			if (res.ok) {
-				const full = await res.json();
-				editingNote = { ...note, backlinks: full.backlinks ?? [] };
-			} else {
-				editingNote = note;
-			}
-		} catch {
-			editingNote = note;
-		}
+	function openEditor(note: Note) {
+		editingNote = note;
 		replaceState(`#${note.id}`, {});
 	}
 
@@ -54,25 +44,18 @@
 		replaceState(location.pathname, {});
 	}
 
-	async function openNoteById(noteId: string) {
-		showNewNote = false;
-		newNoteChecklist = false;
-		editingNote = null;
+	// Reads from the local IndexedDB cache rather than fetching over the network —
+	// this keeps note-link navigation instant and offline-safe (matching how the
+	// rest of the app opens notes), and backlinks are filled in progressively by
+	// NoteEditor's own fetch-if-not-pre-populated effect once the editor is open,
+	// the same pattern already used there for attachments.
+	let openNoteRequestId = 0;
 
-		try {
-			const res = await fetch(`/api/notes/${noteId}`);
-			if (res.ok) {
-				const full = await res.json();
-				editingNote = full;
-				replaceState(`#${noteId}`, {});
-			}
-		} catch {
-			const note = await getIdbNote(noteId);
-			if (note) {
-				editingNote = note;
-				replaceState(`#${noteId}`, {});
-			}
-		}
+	async function openNoteById(noteId: string) {
+		const requestId = ++openNoteRequestId;
+		const note = await getIdbNote(noteId);
+		if (requestId !== openNoteRequestId) return; // a newer navigation superseded this one
+		if (note) openEditor(note);
 	}
 
 	function handleReorder(noteIds: string[]) {
@@ -173,5 +156,13 @@
 {/if}
 
 {#if editingNote}
-	<NoteEditor note={editingNote} onClose={closeEditor} onOpenNote={openNoteById} />
+	<!-- Forces a full destroy+remount on note-switch (e.g. clicking a note-link
+	     while the editor is already open) instead of reusing the same instance
+	     with a new `note` prop — a persisting instance's own back-navigation
+	     effect misreads `openEditor`'s replaceState call as "browser back was
+	     pressed" and closes the editor, and its once-only local state never
+	     resyncs to the new note otherwise. -->
+	{#key editingNote.id}
+		<NoteEditor note={editingNote} onClose={closeEditor} onOpenNote={openNoteById} />
+	{/key}
 {/if}