Replace sponsor program with $VOICEBOX token

- Remove the sponsor feature (sponsors page, homepage promo, footer
  link, Stripe constants, and the app About-page sponsored-by section)
- Add $VOICEBOX token: navbar pill linking to pump.fun and a footer
  Token column with a copyable Solana contract address
- Drop API and Download from the navbar links

app/src/lib/sponsors.ts

@@ -1,10 +0,0 @@
-export type Sponsor = {
-  name: string;
-  url: string;
-  logoSrc: string;
-  logoAlt?: string;
-  /** Set true for solid-black logos that need to flip white in dark mode. */
-  invertOnDark?: boolean;
-};
-
-export const SPONSORS: Sponsor[] = [];