Export notes as Markdown or PDF

The reason to build this is not a missing feature — it is that trusting a
program you maintain alone with years of notes should not mean the notes
can only ever be read by that program.

Reachable from a note's toolbar, a notebook's context menu, and Account →
Alle Notizen exportieren.

Markdown carries over everything with an equivalent: headings, lists,
quotes, code, tables, and task state as `- [x]`. A link to another note in
the same export becomes a relative link to its file; a link to a note
outside it degrades to the plain title rather than leaving a dead
reference. Notes with images or a drawing arrive as a ZIP so the files
travel along, and a multi-note export gets a README index.

PDF goes through the browser's print dialog rather than a bundled PDF
library — the browser already has a good one, and it is the only format
where drawings sit inside the document. Markdown gets them as a PNG
beside the text, with a line saying so, because dropping them silently
would be the wrong kind of quiet.

Two decisions worth recording:

- the ZIP writer is eighty lines here rather than a dependency. Note text
  is small and images are already compressed, so compression bought
  nothing, and stored entries are a handful of fixed-layout records.
- Markdown escaping only covers what changes meaning mid-line, plus a
  leading list or heading marker. Escaping every character Markdown ever
  uses turns a sentence into `Hallo\. Alles gut\!`, which defeats the
  point of choosing a readable format.

42 unit tests on the serialiser and the archive format, plus a browser
suite that unpacks a real download and reads the bytes back.

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

frontend/src/features/notes/EditorPane.jsx

@@ -149,6 +149,7 @@ export function EditorPane({
   onRestore,
   onPermanentDelete,
   onOpenHistory,
+  onExport,
   onCreateNote,
 }) {
   if (!note)
@@ -195,6 +196,9 @@ export function EditorPane({
             minWidth: 0,
           }}
         />
+        <button onClick={onExport} title="Export" style={toolbarButton(T, T.faint)}>
+          {I.export}
+        </button>
         <button onClick={onOpenHistory} title="Version history" style={toolbarButton(T, T.faint)}>
           {I.history}
         </button>