feat: add middleware for pathname header support in @nav fallback

Add Next.js middleware to set x-pathname header on all requests,
enabling Server Components to access pathname for route-based
navigation fallback when @nav slots are not available.

This supports the AppNav component's fallback mechanism for
routes that don't have specific @nav parallel route definitions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/middleware.ts

@@ -0,0 +1,21 @@
+import { NextRequest, NextResponse } from 'next/server'
+
+export function middleware(request: NextRequest) {
+  // Add pathname to headers so Server Components can access it
+  const response = NextResponse.next()
+  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)
+     */
+    '/((?!api|_next/static|_next/image|favicon.ico).*)',
+  ],
+}