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/tailwind.config.js

@@ -31,6 +31,31 @@ export default {
           DEFAULT: 'hsl(var(--danger) / <alpha-value>)',
           foreground: 'hsl(var(--danger-foreground) / <alpha-value>)',
         },
+        // Static brand ramp (from #3ba31f) for utility use (bg-green-500, etc.).
+        // The theme-aware accent above (`primary`) is what most UI should use.
+        green: {
+          50: '#f1f9ec',
+          100: '#dcf0d0',
+          200: '#bde3ab',
+          300: '#93d07d',
+          400: '#63b84a',
+          500: '#3ba31f',
+          600: '#2f8419',
+          700: '#266a15',
+          800: '#1e5312',
+          900: '#163a0e',
+        },
+      },
+      fontFamily: {
+        sans: [
+          'Inter',
+          'ui-sans-serif',
+          'system-ui',
+          '-apple-system',
+          'Segoe UI',
+          'Roboto',
+          'sans-serif',
+        ],
       },
       borderRadius: {
         lg: 'var(--radius)',