fix(api): add global error handler and export app type

- Add .onError hook for centralized error handling and consistent JSON formatting
- Export app instance and App type for Eden Treaty client integration

Reference: Task 2, Issues 1 and 2

app/api/[[...slugs]]/route.ts

@@ -1,7 +1,16 @@
 import { Elysia } from "elysia"
 import { healthRoutes } from "@/lib/api/health"
 
-const app = new Elysia({ prefix: "/api" })
+export const app = new Elysia({ prefix: "/api" })
+  .onError(({ code, error, set, request }) => {
+    console.error(`[API Error] ${code} ${request.url}`,
+      error instanceof Error ? error.message : error
+    )
+    set.status = code === "NOT_FOUND" ? 404 : 500
+    return {
+      error: code === "NOT_FOUND" ? "Not found" : "Internal server error",
+    }
+  })
   .use(healthRoutes)
   .get("/", () => ({
     name: "DeckyVault API",
@@ -13,3 +22,5 @@ export const POST = app.fetch
 export const PUT = app.fetch
 export const DELETE = app.fetch
 export const PATCH = app.fetch
+
+export type App = typeof app