feat: implement 4-theme system with Zustand persistence

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/themes/vibrant.ts

@@ -0,0 +1,31 @@
+export const vibrantTheme = {
+  name: 'vibrant' as const,
+  label: 'Vibrant',
+  layout: 'sidebar-collapsible',
+  cssVars: {
+    '--background': '270 50% 98%',
+    '--foreground': '263.4 84% 6.7%',
+    '--card': '0 0% 100%',
+    '--card-foreground': '263.4 84% 6.7%',
+    '--primary': '262.1 83.3% 57.8%',
+    '--primary-foreground': '210 40% 98%',
+    '--secondary': '187 85.7% 53.3%',
+    '--secondary-foreground': '222.2 47.4% 11.2%',
+    '--muted': '270 30% 94%',
+    '--muted-foreground': '263.4 25% 40%',
+    '--accent': '24.6 95% 53.1%',
+    '--accent-foreground': '0 0% 100%',
+    '--destructive': '0 84.2% 60.2%',
+    '--destructive-foreground': '210 40% 98%',
+    '--success': '142.1 76.2% 36.3%',
+    '--success-foreground': '355.7 100% 97.3%',
+    '--border': '270 30% 88%',
+    '--input': '270 30% 88%',
+    '--ring': '262.1 83.3% 57.8%',
+    '--radius': '1rem',
+  },
+  sidebar: {
+    width: '280px',
+    collapsible: true,
+  },
+};