fix(api): read the error code before the status

A read-failure primitive decided from the status alone, so a 401 from a
step-up or re-proof gate would have read as an ended session and sent
the reader to sign-in. The code now decides first and the status is the
fallback for codes it does not know.

The two invitation routes answer an unknown name or e-mail with
sharing.invite.no_account. Both have a second 404 with a different next
step, and since every error now carries a code the clients match on it
instead of on the status. The status defaults move into a module the
client can import, and the contract text no longer says these arms
carry no code.

src/lib/sharing/grant-view.ts

@@ -112,3 +112,10 @@ export function toGrantView(
     revokedBy: grant.revokedBy,
   };
 }
+
+/**
+ * The 404 both invitation routes answer when no account carries the typed
+ * name or e-mail. Its own code, because each route has a second 404 that
+ * needs a different next step.
+ */
+export const INVITE_NO_ACCOUNT_CODE = "sharing.invite.no_account";