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/auth/SetupAndAuthView.tsx

@@ -14,6 +14,7 @@ import {
 import { AuthStatus, SharingMode, User } from '../../lib/types';
 import { api, triggerHaptic } from '../../lib/api';
 import { ToggleSwitch } from '../../components/ToggleSwitch';
+import { LegalLinks } from '../../components/LegalLinks';
 
 interface SetupAndAuthViewProps {
   authStatus: AuthStatus;
@@ -330,6 +331,7 @@ export const SetupAndAuthView: React.FC<SetupAndAuthViewProps> = ({
             </div>
           </form>
         </div>
+        <LegalLinks legal={authStatus.legal} className="mt-4" />
       </div>
     );
   }
@@ -574,6 +576,7 @@ export const SetupAndAuthView: React.FC<SetupAndAuthViewProps> = ({
           )}
         </div>
       </div>
+      <LegalLinks legal={authStatus.legal} className="mt-4" />
     </div>
   );
 };