Support the phishing-detection experimental client flag

Bitwarden recently integrated a [Phishing Blocker]( feature into its browser extension, warning users when visiting sites listed in the open-source [Phishing.Database]( No Vaultwarden-specific server support is needed beyond exposing the experimental client feature flag phishing-detection. However, since Vaultwarden limits configurable client feature flags to a set hardcoded in [config.rs]( the feature currently cannot be enabled without a custom build. This can be verified by adding phishing-detection to the accepted flag list, starting Vaultwarden with EXPERIMENTAL_CLIENT_FEATURE_FLAGS=phishing-detection, setting up the extension against that instance, and then visiting With that in place, the extension shows the phishing warning as expected. Since this is already an upstream experimental client flag, adding it to Vaultwarden’s allowlist seems like a low-risk improvement that would let self-hosted users opt in without requiring any long-term stability guarantees. <im…