fix: move security headers to middleware composition (qa-requested)

- Move CSP, HSTS, Referrer-Policy, and Permissions-Policy from next.config.ts to middleware
- Compose headers with next-intl middleware to ensure they propagate through rewrites
- Security headers now set via response.headers.set() in middleware after i18n routing
- All security headers should now appear in HTTP responses

Fixes QA issue: headers from next.config.ts not appearing due to middleware rewrites

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

next.config.ts

@@ -42,22 +42,7 @@ const nextConfig: NextConfig = {
             key: 'X-Content-Type-Options',
             value: 'nosniff',
           },
-          {
-            key: 'Content-Security-Policy',
-            value: "default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com data:; img-src 'self' data: blob: https://mxadgucxhmstlzsbgmoz.supabase.co; connect-src 'self' https://mxadgucxhmstlzsbgmoz.supabase.co; frame-ancestors 'self'; base-uri 'self'; form-action 'self'",
-          },
-          {
-            key: 'Strict-Transport-Security',
-            value: 'max-age=31536000; includeSubDomains; preload',
-          },
-          {
-            key: 'Referrer-Policy',
-            value: 'strict-origin-when-cross-origin',
-          },
-          {
-            key: 'Permissions-Policy',
-            value: 'geolocation=(), microphone=(), camera=(), payment=(), usb=()',
-          },
+          // Security headers moved to middleware (CSP, HSTS, Referrer-Policy, Permissions-Policy)
         ],
       },
       {