Sort a notebook, import Markdown back, and stop the page shrinking in Draw

The paper measurably shrank on switching to Draw — 733px to 460px on the same
screen. The editor was a flex item at its default `flex: 0 1 auto`, so its width
was its widest row's: the toolbar. Swapping a dozen formatting buttons for six
drawing ones collapsed it. It now fills the pane, so the width is a property of
the window.

Each notebook remembers how its notes are ordered, on the server rather than per
device. The manual order has to survive being looked away from, so `reindex`
renumbers every note in the notebook rather than the filtered view, and dragging
is switched off while a search narrows the list. Reordering has its own route:
`PUT /notes/:id` would bump `updated_at` and snapshot a version, so a drag would
move a note to the top of "last changed" and spend a slot of its history.

Markdown and our own ZIP archives come back as notes, with their images
re-uploaded as attachments. A drawing returns as the PNG it was exported as —
vectors are not recoverable from pixels — and the import says so rather than
leaving it to be discovered. Writing the parser turned up a real bug in the
exporter: blocks were emitted with no blank line between them, so a list after a
list became one list and a quote after an item was read as part of it.

Creating or importing from "All Notes" now asks which notebook instead of
silently taking the first one. The web-link prompt is a real dialog: a bare
domain becomes https, javascript: is refused with a reason. The PDF viewer's
hide-drawing control is a labelled button rather than a filled-or-hollow dot,
and the export icon points down into a tray like every other download.

186 unit tests, 179 browser checks across eleven suites, including a new
`offline` suite that loads the app cold with the network off.

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

frontend/src/export/markdown.ts

@@ -212,7 +212,19 @@ export function noteToMarkdown(doc: DocNode, options: MarkdownOptions = {}): str
     }
   }
 
-  lines.push(...children(doc, options));
+  /* A blank line between top-level blocks.
+   *
+   * Without one, Markdown runs adjacent blocks together: a bullet list written
+   * straight after another list becomes more items of the first, and a quote or
+   * a code fence on the line after a list item is read as part of that item.
+   * That is not a quirk of one reader — it is what the format says, so the file
+   * rendered wrongly everywhere, and reading it back in produced a document that
+   * did not match the one exported. */
+  const blocks = (doc.content || []).flatMap((n) => {
+    const rendered = block(n, options);
+    return rendered.length && rendered.some((l) => l !== "") ? [...rendered, ""] : [];
+  });
+  lines.push(...blocks);
 
   if (options.hasDrawing) {
     lines.push("", "> ✏️ Diese Notiz enthält eine Zeichnung, die Markdown nicht abbilden kann.");