fix: use project-relative alias strings in default platform config

Why: Turbopack's `resolveAlias` treats absolute paths as server-relative
(prepending `./`) and rejects them with "server relative imports are not
implemented yet". The `fileURLToPath(new URL(...))` pattern produced
absolute paths that broke the plain `pnpm build` CI job. Project-relative
strings resolve correctly in both Turbopack and webpack.

Author: ovflowd

apps/site/next.platform.config.mjs

@@ -1,21 +1,19 @@
-import { fileURLToPath } from 'node:url';
-
 /**
  * Default (no-op) platform config used when no `DEPLOY_TARGET` is set —
  * local dev, static export, generic hosting, etc.
  *
  * Platform deployments (Vercel, Cloudflare, …) provide their own
  * `next.platform.config.mjs` that overrides these values. Keep this
  * file free of any platform-specific code.
+ *
+ * Alias values are project-relative strings (not absolute paths) so
+ * Turbopack resolves them correctly — Turbopack treats absolute paths
+ * as server-relative and rejects them.
  */
 export default {
   aliases: {
-    '@platform/analytics': fileURLToPath(
-      new URL('./platform/analytics.tsx', import.meta.url)
-    ),
-    '@platform/instrumentation': fileURLToPath(
-      new URL('./platform/instrumentation.ts', import.meta.url)
-    ),
+    '@platform/analytics': './platform/analytics.tsx',
+    '@platform/instrumentation': './platform/instrumentation.ts',
   },
   mdx: {
     // Defaults for local dev / static export / generic hosting. Platform