security: add Cloudflare Pages security headers

Add CSP, X-Frame-Options, X-Content-Type-Options, Referrer-Policy,
and Permissions-Policy via _headers file.

Author: hletrd

public/_headers

@@ -0,0 +1,6 @@
+/*
+  X-Content-Type-Options: nosniff
+  X-Frame-Options: DENY
+  Referrer-Policy: strict-origin-when-cross-origin
+  Permissions-Policy: camera=(), microphone=(), geolocation=()
+  Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' https://www.googletagmanager.com https://www.google-analytics.com; connect-src 'self' https://api.burstpick.app https://www.google-analytics.com https://region1.google-analytics.com; img-src 'self' data: https://www.google-analytics.com; style-src 'self' 'unsafe-inline'; font-src 'self'; frame-ancestors 'none'