feat(legal): imprint & privacy notices, "Über MindSift"

- LEGAL_IMPRINT_/LEGAL_PRIVACY_ URL and Markdown (inline or *_FILE);
  only http(s) URLs accepted
- /auth/status exposes legal info; public GET /legal/{imprint|privacy}
- Links on login/setup screen and in the settings footer: URL opens a new
  tab, Markdown a reading modal; hidden when nothing is configured
- "Über MindSift" modal (version, license, repository) in settings and help
- Docs and compose/env examples

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/help/HelpView.tsx

@@ -28,6 +28,7 @@ import {
   Flame,
   Check,
 } from 'lucide-react';
+import { AboutModal } from '../../components/AboutModal';
 
 interface HelpViewProps {
   onBack?: () => void;
@@ -55,6 +56,7 @@ interface DocCard {
 export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
   const [activeCategory, setActiveCategory] = useState<HelpCategory>('all');
   const [searchQuery, setSearchQuery] = useState('');
+  const [showAbout, setShowAbout] = useState(false);
 
   // Documentation sections & cards data
   const cards: DocCard[] = useMemo(
@@ -876,6 +878,13 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
           </div>
         </div>
       )}
+
+      <div className="flex justify-center pt-2">
+        <button type="button" onClick={() => setShowAbout(true)} className="text-[11px] text-muted hover:text-main hover:underline underline-offset-2">
+          Über MindSift
+        </button>
+      </div>
+      {showAbout && <AboutModal onClose={() => setShowAbout(false)} />}
     </div>
   );
 };