fix(behaviours): preserve 'use client' directive in React behaviours build

- Add banner to Vite config to preserve 'use client' directive
- Update examples/nextjs BehavioursLoader with 'use client' directive
- Add comprehensive BEHAVIOURS_ARCHITECTURE.md documentation
- Explains SSR vs Client Component architecture
- Clarifies that BehavioursLoader is ALWAYS a client component
- It enhances SSR HTML after hydration, not during SSR

Fixes 'createContext is not a function' error in Next.js App Router
The directive was being stripped by Vite during build process

Author: Fergus Bisset

config/vite.behaviours.config.ts

@@ -19,6 +19,8 @@ export default defineConfig({
       external: ['react', 'react/jsx-runtime'],
       output: {
         preserveModules: false,
+        // Preserve "use client" directive for Next.js
+        banner: '"use client";',
       },
     },
   },