Unify branding: VT323 logotype across website, TUI, and OAuth pages

- Add VT323 ASCII art logo to logo.mjs as single source of truth
- Website nav and hero use VT323 font via AsciiLogo.astro component
- TUI header and CLI help render the ASCII logo with block-centered alignment
- OAuth callback pages (Pi and alphaXiv) show branded feynman logotype
- Auto-set recommended model after provider login

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/src/components/AsciiLogo.astro

@@ -7,13 +7,13 @@ interface Props {
 const { class: className = '', size = 'hero' } = Astro.props;
 
 const sizeClasses = size === 'nav'
-  ? 'text-lg'
-  : 'text-4xl sm:text-5xl md:text-6xl';
+  ? 'text-2xl'
+  : 'text-6xl sm:text-7xl md:text-8xl';
 ---
 
 <span
   class:list={[
-    "font-['Silkscreen'] text-accent font-bold tracking-tight inline-block",
+    "font-['VT323'] text-accent inline-block tracking-tighter",
     sizeClasses,
     className,
   ]}