feat: add theme-based layouts

Each theme now has a unique layout:
- Light: Standard fixed sidebar
- Vibrant: Horizontal top navigation
- Corporate: Compact sidebar (expands on hover)
- Dark: Floating sidebar with glass effect

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

apps/web/themes/vibrant.ts

@@ -1,7 +1,7 @@
 export const vibrantTheme = {
   name: 'vibrant' as const,
   label: 'Vibrant',
-  layout: 'sidebar-collapsible',
+  layout: 'topnav' as const,
   cssVars: {
     '--background': '270 50% 98%',
     '--foreground': '263.4 84% 6.7%',
@@ -24,8 +24,4 @@ export const vibrantTheme = {
     '--ring': '262.1 83.3% 57.8%',
     '--radius': '1rem',
   },
-  sidebar: {
-    width: '280px',
-    collapsible: true,
-  },
 };