feat(login): Sign in with Zitadel (#12530)

Closes #11824

# Which Problems Are Solved

The backend supports the dedicated ZITADEL identity provider template
(`AddZitadelProvider`, `IDENTITY_PROVIDER_TYPE_ZITADEL`), but the new
login (login v2)
cannot yet handle providers of this type:

A configured and activated ZITADEL IdP is not rendered as a sign-in
option, because the provider type is missing from the login's component
mapping.

# How the Problems Are Solved

- Regenerated `@zitadel/proto` (`settings/v2/login_settings_pb` in
`types`, `es` and `cjs`)
so `IdentityProviderType.ZITADEL = 13` is available (`idp/v2` was
already up to date).
- `apps/login/src/lib/idp.ts`:
- `idpTypeToSlug` maps `ZITADEL` to the `zitadel` slug, used by the
generic
    `/idp/[provider]/process|failure` routes (no route changes needed).
  - `idpTypeToIdentityProviderType` maps `IDPType.IDP_TYPE_ZITADEL` to
    `IdentityProviderType.ZITADEL`.
- Added `SignInWithZitadel` button
(`apps/login/src/components/idps/sign-in-with-zitadel.tsx`)
with the Zitadel logo as inline SVG: the brand gradient is
theme-independent, accents use
`currentColor` to adapt to light/dark, consistent with the other
provider buttons.
- Registered the Zitadel type in the provider-to-component mapping in
`sign-in-with-idp.tsx`.

No changes to the IdP intent handling are required: user
auto-creation/auto-update consume
the pre-mapped `user_action.create_user` / `update_user` from
`RetrieveIdentityProviderIntent`,
which the backend fills for the Zitadel provider like for any generic
OIDC provider.

# Additional Changes

- Added the `signInWithZitadel` i18n key ("Sign in with Zitadel") to all
15 locale files.
- Extended `idp.test.ts` with the new slug and type mappings and added
the Zitadel type to
  the exhaustive enum-coverage tests (45 tests passing).

# Additional Context

- Part of the "Zitadel as an Identity Provider" epic: #5127
- Enables the customer-portal "support access" flow, where support staff
sign in to a
  customer instance via a pre-configured Zitadel Support IdP

---------

Co-authored-by: Copilot Autofix powered by AI <[email protected]>

internal/api/idp/idp.go

@@ -35,6 +35,7 @@ import (
 	"github.com/zitadel/zitadel/internal/idp/providers/oauth"
 	openid "github.com/zitadel/zitadel/internal/idp/providers/oidc"
 	saml2 "github.com/zitadel/zitadel/internal/idp/providers/saml"
+	"github.com/zitadel/zitadel/internal/idp/providers/zitadel"
 	"github.com/zitadel/zitadel/internal/query"
 	"github.com/zitadel/zitadel/internal/zerrors"
 )
@@ -555,6 +556,8 @@ func (h *Handler) fetchIDPUserFromCode(ctx context.Context, identityProvider idp
 		session = openid.NewSession(provider.Provider, code, idpArguments)
 	case *google.Provider:
 		session = openid.NewSession(provider.Provider, code, idpArguments)
+	case *zitadel.Provider:
+		session = openid.NewSession(provider.Provider, code, idpArguments)
 	case *apple.Provider:
 		session = apple.NewSession(provider, code, appleUser)
 	case *jwt.Provider, *ldap.Provider, *saml2.Provider: