feat: fix signature editor init and expand README

- Pass initialContent prop to WysiwygEditor instead of imperative setContent
  to ensure mail signature loads correctly on mount
- Rewrite README to reflect full project scope (scanner inbox, email import,
  postprocessing rules, label printing, OIDC auth)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-frontend/src/pages/UserSettingsPage.tsx

@@ -68,6 +68,7 @@ function MailSettingsTab() {
   const [testing, setTesting] = useState(false);
   const [testResult, setTestResult] = useState<{ ok: boolean; error?: string } | null>(null);
   const [passSet, setPassSet] = useState(false);
+  const [signatureHtml, setSignatureHtml] = useState<string | null>(null);
   const editorRef = useRef<WysiwygEditorHandle>(null);
 
   useEffect(() => {
@@ -81,9 +82,7 @@ function MailSettingsTab() {
         smtpFromName: data.smtpFromName ?? '',
       });
       setPassSet(data.smtpPassSet);
-      if (data.mailSignatureHtml) {
-        editorRef.current?.setContent(data.mailSignatureHtml);
-      }
+      setSignatureHtml(data.mailSignatureHtml ?? null);
     }).catch(() => {
       message.error('Einstellungen konnten nicht geladen werden');
     }).finally(() => setLoading(false));
@@ -167,7 +166,7 @@ function MailSettingsTab() {
       </Form.Item>
 
       <Form.Item label="Signatur">
-        <WysiwygEditor ref={editorRef} minHeight={180} />
+        <WysiwygEditor ref={editorRef} initialContent={signatureHtml ?? ''} minHeight={180} />
       </Form.Item>
 
       <Form.Item>