chore: send security headers with every response

_headers

@@ -0,0 +1,22 @@
+# Response headers for everything flield.com serves. Cloudflare reads this
+# file at deploy time and does not serve it. Every page is static, has no
+# form, and talks to nothing but its own origin, so the policy can say
+# exactly that.
+#
+# The one exception is Cloudflare Web Analytics, which the dashboard
+# injects at the edge as a script from static.cloudflareinsights.com that
+# then posts to cloudflareinsights.com. Both hosts are allowed below; drop
+# them if analytics is ever turned off.
+#
+# script-src carries 'unsafe-inline' because the app is one long inline
+# script and the document pages each have one too. A nonce would need a
+# server to mint it, and there is none. What the policy still buys is that
+# no script from any other host runs, no page can be re-based, nothing
+# embeds a plugin, and no form posts anywhere. img-src allows data: for
+# the favicon the app redraws on every load and blob: for the exported
+# frames; worker-src covers the vendored GIF encoder's worker.
+/*
+  X-Content-Type-Options: nosniff
+  Referrer-Policy: strict-origin-when-cross-origin
+  Permissions-Policy: camera=(), microphone=(), geolocation=(), payment=(), usb=()
+  Content-Security-Policy: default-src 'self'; script-src 'self' 'unsafe-inline' https://static.cloudflareinsights.com; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob:; worker-src 'self' blob:; connect-src 'self' https://cloudflareinsights.com; font-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'