refactor: rename middleware to proxy for Next 16

Next 16 deprecates the middleware file convention; a leftover
middleware.js is silently ignored, which would break the x-pathname
header that canonical URL generation depends on.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

proxy.js

@@ -0,0 +1,24 @@
+import { NextResponse } from 'next/server';
+
+export function proxy(request) {
+    const response = NextResponse.next();
+
+    // Add pathname to headers so we can access it in getCanonicalUrl
+    response.headers.set('x-pathname', request.nextUrl.pathname);
+
+    return response;
+}
+
+export const config = {
+    matcher: [
+        /*
+         * Match all request paths except for the ones starting with:
+         * - api (API routes)
+         * - _next/static (static files)
+         * - _next/image (image optimization files)
+         * - favicon.ico (favicon file)
+         * - panel, dashboard, login, login-verify (auth-gated/noindex pages — canonical URL has no SEO value here)
+         */
+        '/((?!api|_next/static|_next/image|favicon.ico|panel|dashboard|login|login-verify).*)',
+    ],
+};