feat: implement 4-theme system with Zustand persistence

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

apps/web/themes/dark.ts

@@ -0,0 +1,36 @@
+export const darkTheme = {
+  name: 'dark' as const,
+  label: 'Dark',
+  layout: 'minimal-floating',
+  cssVars: {
+    '--background': '0 0% 3.9%',
+    '--foreground': '0 0% 98%',
+    '--card': '0 0% 6%',
+    '--card-foreground': '0 0% 98%',
+    '--primary': '187.2 85.7% 53.3%',
+    '--primary-foreground': '0 0% 3.9%',
+    '--secondary': '0 0% 12%',
+    '--secondary-foreground': '0 0% 98%',
+    '--muted': '0 0% 12%',
+    '--muted-foreground': '0 0% 63.9%',
+    '--accent': '142.1 70.6% 45.3%',
+    '--accent-foreground': '0 0% 3.9%',
+    '--destructive': '0 62.8% 30.6%',
+    '--destructive-foreground': '0 0% 98%',
+    '--success': '142.1 70.6% 45.3%',
+    '--success-foreground': '144.9 80.4% 10%',
+    '--border': '0 0% 14.9%',
+    '--input': '0 0% 14.9%',
+    '--ring': '187.2 85.7% 53.3%',
+    '--radius': '0.75rem',
+  },
+  sidebar: {
+    width: '64px',
+    collapsible: false,
+    iconsOnly: true,
+  },
+  effects: {
+    blur: '10px',
+    glow: '0 0 20px rgba(34,211,238,0.3)',
+  },
+};