feat: implement 4-theme system with Zustand persistence

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

apps/web/themes/light.ts

@@ -0,0 +1,31 @@
+export const lightTheme = {
+  name: 'light' as const,
+  label: 'Light',
+  layout: 'sidebar-fixed',
+  cssVars: {
+    '--background': '0 0% 100%',
+    '--foreground': '222.2 84% 4.9%',
+    '--card': '0 0% 100%',
+    '--card-foreground': '222.2 84% 4.9%',
+    '--primary': '221.2 83.2% 53.3%',
+    '--primary-foreground': '210 40% 98%',
+    '--secondary': '210 40% 96.1%',
+    '--secondary-foreground': '222.2 47.4% 11.2%',
+    '--muted': '210 40% 96.1%',
+    '--muted-foreground': '215.4 16.3% 46.9%',
+    '--accent': '210 40% 96.1%',
+    '--accent-foreground': '222.2 47.4% 11.2%',
+    '--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': '214.3 31.8% 91.4%',
+    '--input': '214.3 31.8% 91.4%',
+    '--ring': '221.2 83.2% 53.3%',
+    '--radius': '0.5rem',
+  },
+  sidebar: {
+    width: '240px',
+    collapsible: false,
+  },
+};