fix: serve Anthropic fonts locally instead of from external CDN

HuggingFace CSP may block cross-origin font loading. Downloaded all three
font files (Anthropic Sans Roman/Italic + Anthropic Mono, ~308KB total) to
ui/public/fonts/ so they're served from the same domain as the app.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/index.css

@@ -10,21 +10,21 @@
    ════════════════════════════════════════════════════════════════════════ */
 @font-face {
   font-family: "Anthropic Sans";
-  src: url(https://cdn.prod.website-files.com/67ce28cfec624e2b733f8a52/69971a00a3295036497e1a28_AnthropicSans-Roman-Web.woff2) format("woff2");
+  src: url(/fonts/69971a00a3295036497e1a28_AnthropicSans-Roman-Web.woff2) format("woff2");
   font-weight: 300 800;
   font-style: normal;
   font-display: swap;
 }
 @font-face {
   font-family: "Anthropic Sans";
-  src: url(https://cdn.prod.website-files.com/67ce28cfec624e2b733f8a52/69971a016067bf14b9b8f48d_AnthropicSans-Italic-Web.woff2) format("woff2");
+  src: url(/fonts/69971a016067bf14b9b8f48d_AnthropicSans-Italic-Web.woff2) format("woff2");
   font-weight: 300 800;
   font-style: italic;
   font-display: swap;
 }
 @font-face {
   font-family: "Anthropic Mono";
-  src: url(https://cdn.prod.website-files.com/67ce28cfec624e2b733f8a52/69971a2e55d24d61bc045b1a_AnthropicMono-Roman-Web.woff2) format("woff2");
+  src: url(/fonts/69971a2e55d24d61bc045b1a_AnthropicMono-Roman-Web.woff2) format("woff2");
   font-weight: 300 800;
   font-style: normal;
   font-display: swap;