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/pages/index.astro

@@ -1,10 +1,12 @@
 ---
 import Base from '../layouts/Base.astro';
+import AsciiLogo from '../components/AsciiLogo.astro';
 ---
 
 <Base title="Feynman — The open source AI research agent" active="home">
   <section class="text-center pt-24 pb-20 px-6">
     <div class="max-w-2xl mx-auto">
+      <AsciiLogo size="hero" class="mb-4" />
       <h1 class="text-5xl sm:text-6xl font-bold tracking-tight mb-6" style="text-wrap: balance">The open source AI research agent</h1>
       <p class="text-lg text-text-muted mb-10 leading-relaxed" style="text-wrap: pretty">Investigate topics, write papers, run experiments, review research, audit codebases &mdash; every output cited and source-grounded</p>
       <div class="inline-flex items-center gap-3 bg-surface rounded-lg px-5 py-3 mb-8 font-mono text-sm">