feat(ui): add desktop footer, legal custom link and about dialog

frontend/src/lib/types.ts

@@ -20,6 +20,8 @@ export interface AuthStatus {
     hasImprintContent?: boolean;
     privacyUrl?: string;
     hasPrivacyContent?: boolean;
+    customLinkLabel?: string;
+    customLinkUrl?: string;
   };
   // Voice input: own Whisper server, the browser's recognition, or switched off (STT_* environment)
   voice?: { mode: VoiceMode; maxSeconds: number };