feat(auth): registration mode on ConnectPage (PUBLIC_ENABLE_REGISTRATION)

Add a login/register toggle to ConnectPage backed by a new
useRegisterMutation (register -> /auth/me, mirroring login). The toggle
is shown only when REGISTRATION_ENABLED, resolved with the same
precedence as the API base URL: runtime window.__APP_CONFIG__ >
PUBLIC_ENABLE_REGISTRATION env > default true. The prod runtime-config
script injects the runtime flag. The backend's ALLOW_REGISTRATION stays
the real authority; this only gates the UI. EN/RU strings added.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/env.d.ts

@@ -1,6 +1,7 @@
 /// <reference types="@rsbuild/core/types" />
 interface ImportMetaEnv {
   readonly PUBLIC_API_BASE_URL?: string;
+  readonly PUBLIC_ENABLE_REGISTRATION?: string;
 }
 interface ImportMeta {
   readonly env: ImportMetaEnv;
@@ -11,5 +12,6 @@ interface ImportMeta {
 interface Window {
   __APP_CONFIG__?: {
     apiBaseUrl?: string;
+    enableRegistration?: boolean;
   };
 }