fix: schemas Sanity en plain objects + config images CDN

- Retire embedded Sanity Studio (incompatible React 18)
- Schemas en plain JS pour documentation/copie vers Studio séparé
- Ajoute config images Sanity CDN dans next.config.ts

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

sanity/schemas/index.ts

@@ -1,12 +1,2 @@
-// Sanity Schema Index
-// Importez ces schémas dans votre Sanity Studio (sanity.config.ts)
-//
-// import portfolio from './schemas/portfolio'
-// import siteSettings from './schemas/siteSettings'
-//
-// export default defineConfig({
-//   schema: { types: [portfolio, siteSettings] }
-// })
-
-export { default as portfolio } from "./portfolio";
-export { default as siteSettings } from "./siteSettings";
+export { portfolioSchema } from "./portfolio";
+export { siteSettingsSchema } from "./siteSettings";