chore: add public assets and deployment configuration

- Add _headers for security headers and asset caching
- Add robots.txt for SEO with sitemap reference
- Add _redirects for SPA client-side routing support

public/_headers

@@ -0,0 +1,9 @@
+/*
+  X-Frame-Options: DENY
+  X-Content-Type-Options: nosniff
+  Referrer-Policy: strict-origin-when-cross-origin
+  Permissions-Policy: camera=(), microphone=(), geolocation=()
+
+/assets/*
+  Cache-Control: public, max-age=31536000, immutable
+

public/_redirects

@@ -0,0 +1,3 @@
+# SPA redirect for client-side routing
+/*    /index.html   200
+

public/robots.txt

@@ -0,0 +1,5 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://xn--bris-v0b.com/sitemap.xml
+