[codex] Add configurable native Referer header

[riderx]

Summary

• add a server.referer config option to Capacitor declarations
• apply the configured Referer as a default header for native HTTP requests on Android and iOS
• preserve per-request Referer headers and ignore invalid or non-HTTP(S) values
• cover the config parsing and header behavior with platform tests

Why

Capacitor already supports a config-driven native user agent override, but there is no equivalent for Referer. Some integrations require a valid HTTP Referer for native requests while still allowing request-specific headers to win.

Closes #3210

Validation

• npm run lint
• npm --workspace android run verify
• xcodebuild clean build -workspace Capacitor.xcworkspace -scheme Capacitor -sdk iphonesimulator -destination 'generic/platform=iOS Simulator'
• xcodebuild clean build -scheme Cordova -sdk iphonesimulator -destination 'generic/platform=iOS Simulator'