feat: implement 4-theme system with Zustand persistence

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

apps/web/themes/index.ts

@@ -0,0 +1,16 @@
+import { lightTheme } from './light';
+import { vibrantTheme } from './vibrant';
+import { corporateTheme } from './corporate';
+import { darkTheme } from './dark';
+
+export const themes = {
+  light: lightTheme,
+  vibrant: vibrantTheme,
+  corporate: corporateTheme,
+  dark: darkTheme,
+} as const;
+
+export type ThemeName = keyof typeof themes;
+export type Theme = (typeof themes)[ThemeName];
+
+export { lightTheme, vibrantTheme, corporateTheme, darkTheme };