Support `undetermined-cipher-scenario-logic` feature flag

Summary Vaultwarden's SUPPORTED_FEATURE_FLAGS whitelist ([src/config.rs#L1429-L1432]( does not include undetermined-cipher-scenario-logic, so self-hosted browser-extension users cannot opt in to Bitwarden's rewritten post-submit add/update login notification logic. Context The flag was introduced in [bitwarden/clients#18395]( (merged 2026-02-02, first shipped in Browser 2026.2.0) and gates the new notification triggering logic that defers cipher-type inference until the vault is unlocked, replacing the separate Change/Add scenarios with a unified Cipher scenario. The Bitwarden maintainer described the rewrite as making add/update notifications "more reliably/consistently" work ([bitwarden/clients#17101]( comment 2026-02-23, closed as duplicate of #17089). The official Bitwarden server exposes the flag ([src/Core/Constants.cs#L173-L176]( Autofill Team), and the client-side default is FALSE ([enum #L30]( [default #L143]( so the new logic only runs when the server…