fix: resolve Next.js 16 build errors

- Extract Elysia app from route.ts into lib/api/app.ts to avoid
  Next.js 16 invalid export validation ('app' is not a valid route
  export field)
- Remove runtime='nodejs' from opengraph-image.tsx (invalid segment
  config in Next.js 16)
- Replace computed revalidate expression in sitemap.ts with literal
  (Next.js cannot statically evaluate BinaryExpression for segment
  configs)
- Update eden.ts imports to match new app.ts location

lib/eden.ts

@@ -1,5 +1,5 @@
 import { treaty } from "@elysia/eden"
-import type { App } from "@/app/api/[[...slugs]]/route"
+import type { App } from "@/lib/api/app"
 
 // Use require() on the server branch to prevent the client bundler from
 // pulling in Elysia and the route module. The typeof window check is the
@@ -7,5 +7,5 @@ import type { App } from "@/app/api/[[...slugs]]/route"
 export const api =
   typeof window === "undefined"
     ? // eslint-disable-next-line @typescript-eslint/no-require-imports
-      treaty((require("../app/api/[[...slugs]]/route") as { app: App }).app).api
+      treaty((require("@/lib/api/app") as { app: App }).app).api
     : treaty<App>(process.env.NEXT_PUBLIC_SITE_URL || "http://localhost:3000").api