feat: replace Tailwind with UnoCSS

Faster dev HMR, native Astro 6 support, same utility class names.
Tokens migrated to uno.config.ts.

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

uno.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig, presetWind } from 'unocss'
+
+export default defineConfig({
+  presets: [presetWind()],
+  theme: {
+    colors: {
+      brand: '#f28a4b',
+      'text-primary': '#2e2a28',
+      'text-secondary': '#7a726d',
+      surface: '#f8f3ee',
+      'surface-alt': '#f2eae3',
+      'surface-footer': '#efe6de',
+      'border-light': '#e3d9d1',
+    },
+    fontFamily: {
+      sans: ['Inter', 'sans-serif'],
+    },
+  },
+})