Migrate CSP middleware to Next.js proxy convention.

Renames src/middleware.ts to src/proxy.ts per Next.js 16 deprecation guidance. CSP nonce behavior is unchanged; build no longer emits the middleware warning.

Co-authored-by: Cursor <cursoragent@cursor.com>

desktop-shell/tauri-skeleton/scripts/build-frontend-export.cjs

@@ -17,7 +17,7 @@ const excludedPaths = [
   '.next',
   'out',
   'src/app/api',
-  'src/middleware.ts',
+  'src/proxy.ts',
 ];
 
 function normalizeRelativePath(target) {