feat(ui): design-system v2 tokens — green accent, warm neutrals, dark-first

Re-point the design tokens to the approved brief (docs/discovery/04):
- green brand accent from #3ba31f (contrast-tuned per theme; white-on-green AA)
- warm-leaning neutrals; genuine white light theme; warm-charcoal dark (not pure black)
- 8px base radius; Inter-preferred font stack; static green ramp for utilities
- dark-first default for new users
Also re-skins the legacy styles.css palette so the current app adopts the brand.
Verified: frontend builds clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/index.html

@@ -9,8 +9,8 @@
       // Apply the saved theme before first paint to avoid a flash of the wrong mode.
       (function () {
         try {
-          var t = localStorage.getItem('ii:theme');
-          if (!t) t = matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+          // Dark-first: default new users to dark unless they've chosen light.
+          var t = localStorage.getItem('ii:theme') || 'dark';
           if (t === 'dark') document.documentElement.classList.add('dark');
         } catch (e) {}
       })();