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/ui/AccountMenu.jsx

@@ -3,7 +3,7 @@ import * as sync from "../syncEngine";
 import { I } from "./icons.jsx";
 
 /* ═══════ ACCOUNT MENU ═══════ */
-export function AccountMenu({ user, onClose, onLogout, onUserUpdate, onOpenAdmin, dark }) {
+export function AccountMenu({ user, onClose, onLogout, onUserUpdate, onOpenAdmin, onExportAll, dark }) {
   const [tab, setTab] = useState("profile"); // profile | username | password
   const [displayName, setDisplayName] = useState(user?.displayName || "");
   const [newUsername, setNewUsername] = useState(user?.username || "");
@@ -367,6 +367,27 @@ export function AccountMenu({ user, onClose, onLogout, onUserUpdate, onOpenAdmin
           )}
         </div>
         <div style={{ padding: "0 22px 18px" }}>
+          <button
+            onClick={onExportAll}
+            style={{
+              display: "flex",
+              alignItems: "center",
+              gap: 8,
+              width: "100%",
+              padding: "10px",
+              borderRadius: 10,
+              border: `1px solid ${T.border}`,
+              background: "transparent",
+              cursor: "pointer",
+              color: T.muted,
+              fontSize: 13,
+              fontWeight: 600,
+              justifyContent: "center",
+              marginBottom: 8,
+            }}
+          >
+            {I.export} Alle Notizen exportieren
+          </button>
           {user?.isAdmin && (
             <button
               onClick={onOpenAdmin}