feat(auth): recover cross-signal authorization with provision secrets

A client that could not authorize against a foreign signal server was
redirected into a dead end with no way to retry, so servers joined from
another signal route became unreachable.

The home server now stores a per-user provision secret, clients keep it in
their own store, and a recovery service records why authorization failed per
server URL. Invite, server browser, and chat room surface that reason and
offer a retry instead of silently redirecting.

toju-app/public/i18n/catalog/auth.json

@@ -37,6 +37,11 @@
       "defaultServerName": "Signal Server"
     },
     "provision": {
+      "credentialsRejected": "This server already has an account that the restored session cannot unlock. Your home account is still signed in.",
+      "reconnectTitle": "Reconnect to {{serverName}}",
+      "retry": "Retry",
+      "retrying": "Retrying…",
+      "serverUnavailable": "This server is currently unavailable. Retry when the connection is restored.",
       "usernameCollision": "Username {{preferredUsername}} was taken on {{serverName}}. Created {{provisionedUsername}} instead."
     }
   }