Ask what a single note's export should contain, and fix drawings never being in one

Two questions instead of one, because the answers depend on each other:
Markdown holds text only, a PNG holds the drawing only, PDF holds both.
Listing every format flat would have offered combinations that cannot
exist. A note without a drawing skips the first step rather than showing
it mostly greyed out.

While testing that, the drawings turned out never to have made it into
any export at all — not the ZIP, not the PDF. strokeDocOf() *builds* a
stroke document from a bare array, and it was being handed one that was
already a document, so the strokes ended up nested one level too deep and
every length check read undefined. Nothing complained; the export simply
came out without the drawing. It went unnoticed because the notes the
earlier suite exported had none.

Now uses docStrokes(), and the suite draws on a note before exporting it,
checks the PNG's magic bytes, and checks it is 159 KB rather than an empty
white page — a check on the file merely existing would have passed against
the broken version too.

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

frontend/src/export/exportNotes.ts

@@ -10,7 +10,7 @@
 import * as sync from "../syncEngine";
 import { parseNoteContent } from "../editor/content";
 import { parsingExtensions } from "../editor/extensions.js";
-import { CANON_W, CANON_H, paintStrokeDoc, sizeCanvas, strokeDocOf } from "../features/drawing/strokes.js";
+import { CANON_W, CANON_H, docStrokes, paintStrokeDoc, sizeCanvas } from "../features/drawing/strokes.js";
 import type { NoteContent, NoteMeta, Notebook } from "../types";
 import { noteToMarkdown, safeFilename } from "./markdown";
 import { printNotes, type PrintableNote } from "./print";
@@ -30,9 +30,10 @@ type Loaded = { meta: NoteMeta; content: NoteContent };
 /* ── pieces ── */
 
 /** Renders a stroke document to PNG bytes, so a drawing survives as a picture. */
-async function drawingToPng(strokes: unknown): Promise<Uint8Array | null> {
-  const doc = strokeDocOf(strokes);
-  if (!doc?.strokes?.length) return null;
+async function drawingToPng(doc: unknown): Promise<Uint8Array | null> {
+  // `doc` is already a stroke document as stored — strokeDocOf() builds one from
+  // a bare array and would wrap this one a second time, hiding the strokes.
+  if (docStrokes(doc).length === 0) return null;
 
   const canvas = document.createElement("canvas");
   const height = Math.round((DRAWING_EXPORT_W * CANON_H) / CANON_W);
@@ -81,10 +82,31 @@ function nameFor(meta: NoteMeta, taken: Set<string>): string {
 
 /* ── single note ── */
 
+/** What a single-note export should contain. */
+export type ExportParts = "text" | "drawing" | "both";
+
+/** Whether this note has a drawing — decides which options the wizard offers. */
+export async function noteHasDrawing(meta: NoteMeta): Promise<boolean> {
+  try {
+    const { content } = await loadNote(meta);
+    return docStrokes(content.drawingStrokes).length > 0;
+  } catch {
+    return false;
+  }
+}
+
+/** The drawing on its own, as a PNG on white paper. */
+export async function exportDrawingAsPng(meta: NoteMeta): Promise<void> {
+  const { content } = await loadNote(meta);
+  const png = await drawingToPng(content.drawingStrokes);
+  if (!png) throw new Error("Diese Notiz enthält keine Zeichnung.");
+  download(bytesToBlob(png, "image/png"), `${safeFilename(meta.title)}.png`);
+}
+
 export async function exportNoteAsMarkdown(meta: NoteMeta): Promise<void> {
   const { content } = await loadNote(meta);
   const doc = parseNoteContent(content.content, parsingExtensions);
-  const hasDrawing = !!strokeDocOf(content.drawingStrokes)?.strokes?.length;
+  const hasDrawing = docStrokes(content.drawingStrokes).length > 0;
   const ids = attachmentIds(doc);
 
   const md = noteToMarkdown(doc, {
@@ -133,7 +155,8 @@ const blobToDataUrl = (blob: Blob): Promise<string> =>
 export async function exportNotesAsPdf(
   metas: NoteMeta[],
   title: string,
-  onProgress?: (p: ExportProgress) => void
+  onProgress?: (p: ExportProgress) => void,
+  parts: ExportParts = "both"
 ): Promise<void> {
   const printable: PrintableNote[] = [];
   const total = metas.length;
@@ -153,10 +176,10 @@ export async function exportNotesAsPdf(
         }
       }
 
-      const png = await drawingToPng(content.drawingStrokes);
+      const png = parts === "text" ? null : await drawingToPng(content.drawingStrokes);
       const drawing = png ? await blobToDataUrl(bytesToBlob(png, "image/png")) : null;
 
-      printable.push({ meta, content, drawing, images });
+      printable.push({ meta, content, drawing, images, includeText: parts !== "drawing" });
     } catch {
       /* an unreadable note is left out rather than aborting the whole print */
     }
@@ -225,7 +248,7 @@ export async function exportNotesAsZip({
     try {
       const { content } = await loadNote(meta);
       const doc = parseNoteContent(content.content, parsingExtensions);
-      const hasDrawing = !!strokeDocOf(content.drawingStrokes)?.strokes?.length;
+      const hasDrawing = docStrokes(content.drawingStrokes).length > 0;
 
       entries.push({
         name: `${name}.md`,