style: switch to Geist + Geist Mono fonts, add antialiasing

Replaces Space Grotesk with Geist (Vercel's font, built for developer UIs)
and JetBrains Mono with Geist Mono for a cohesive, premium-grade look.
Geist is significantly more refined at small sizes — tighter spacing,
neutral geometry, and designed from the ground up for dark UI contexts.

Also adds -webkit-font-smoothing: antialiased and letter-spacing: -0.01em
which are the primary reasons premium SaaS fonts look sharp vs flat.

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

ui/src/index.css

@@ -8,7 +8,7 @@
            Warmer and earthier than amber, more "drawn" than digital.
    Every surface is slightly warm — no cold values anywhere.
    ════════════════════════════════════════════════════════════════════════ */
-@import url('https://fonts.googleapis.com/css2?family=Space+Grotesk:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;500;600;700&display=swap');
+@import url('https://fonts.googleapis.com/css2?family=Geist:wght@300;400;500;600;700&family=Geist+Mono:wght@400;500;600&display=swap');
 
 *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
 html { height: 100dvh; background: #1A1916; overflow: hidden; }
@@ -62,8 +62,8 @@ html { height: 100dvh; background: #1A1916; overflow: hidden; }
   --pill-bg: var(--surface-3);
 
   /* Typography */
-  --sans: "Space Grotesk", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
-  --mono: "JetBrains Mono", "Fira Code", "Cascadia Code", monospace;
+  --sans: "Geist", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
+  --mono: "Geist Mono", "JetBrains Mono", "Fira Code", monospace;
 
   /* Spacing (8px grid) */
   --space-1: 4px;  --space-2: 8px;  --space-3: 12px;
@@ -103,6 +103,9 @@ body {
   color: var(--text);
   height: 100%;
   overflow: hidden;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+  letter-spacing: -0.01em;
   /* Deep atmospheric bloom — LiteLLM style: bold primary bloom, counter-bloom, center warmth */
   background-image:
     radial-gradient(ellipse 70% 55% at -5% 0%,   rgba(180, 80, 40, 0.32) 0%, transparent 65%),