Make `suppressOnboardingInterstitials` configurable

GET /api/config currently returns a hardcoded value: // src/api/core/mod.rs, fn config() "settings": { "disableUserRegistration": CONFIG.is_signup_disabled(), "suppressOnboardingInterstitials": false }, Upstream Bitwarden treats this as a regular admin setting, right next to the one Vaultwarden already wires up: // bitwarden/server, src/Core/Settings/GlobalSettings.cs public virtual bool DisableUserRegistration { get; set; } public virtual bool SuppressOnboardingInterstitials { get; set; } Both are passed through side by side in ConfigResponseModel.cs, so the pair is symmetric upstream — only one half is configurable here. Per the comment in Vaultwarden's own source, enabling it suppresses post-login welcome dialogs, extension install prompts, setup-extension redirects and premium upsell modals. On a small self-hosted instance where everyone is onboarded by hand, those prompts are pure…