Remove App Router 404s and reduce per-request allocations (#60410)

Remove unused App Router (26 files, ~1,500 lines) and eliminate per-request shallow-copy allocations from memoized data reads. Fix learning-track code to copy-before-mutate instead. Skip redundant fs.stat in production. Update 404 tests for minimal HTML response.

Author: heiskr

next.config.ts

@@ -36,13 +36,7 @@ const config: NextConfig = {
   },
   sassOptions: {
     quietDeps: true,
-    silenceDeprecations: [
-      'legacy-js-api',
-      'import',
-      'global-builtin',
-      'color-4-api',
-      'mixed-decls',
-    ],
+    silenceDeprecations: ['legacy-js-api', 'import', 'global-builtin', 'color-4-api'],
   },
   // Don't use automatic Next.js logging in dev unless the log level is `debug` or higher
   // See `src/observability/logger/README.md` for log levels