refactor: use 'c' alias for constants package

- Update all imports from 'constants' to 'c' for brevity
- Replace all 'constants.' references with 'c.'
- Fix remaining hardcoded content-type headers in httputil
- Fix remaining hardcoded User-Agent and Accept headers
- Rename CSRF receiver from 'c' to 'csrf' to avoid conflict
- Add ContentTypePlainSimple constant for Accept header matching
- Fix JSONCached to use proper integer formatting

internal/constants/constants.go

@@ -12,6 +12,7 @@ const (
 	ContentTypeHTML         = "text/html; charset=utf-8"
 	ContentTypeHTMLFragment = "text/html" // For HTMX fragments
 	ContentTypePlainText    = "text/plain; charset=utf-8"
+	ContentTypePlainSimple  = "text/plain" // For Accept header matching
 	ContentTypePDF          = "application/pdf"
 	ContentTypeFormURLEnc   = "application/x-www-form-urlencoded"
 )