fix: allow R2 assets in Content-Security-Policy img-src

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ferr079

public/_headers

@@ -5,4 +5,4 @@
   Referrer-Policy: strict-origin-when-cross-origin
   Permissions-Policy: camera=(), microphone=(), geolocation=(), interest-cohort=()
   Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
-  Content-Security-Policy: default-src 'none'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' https://cdn.simpleicons.org data:; font-src 'self'; connect-src 'self'; frame-ancestors 'none'; base-uri 'self'; form-action 'self'
+  Content-Security-Policy: default-src 'none'; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; img-src 'self' https://cdn.simpleicons.org https://assets.pixelium.win data:; font-src 'self'; connect-src 'self'; frame-ancestors 'none'; base-uri 'self'; form-action 'self'