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/api/types.ts

@@ -113,6 +113,11 @@ export interface LoginResponse {
   tokens: AuthTokens;
 }
 
+export interface RegisterRequest {
+  username: string;
+  password: string;
+}
+
 export interface PaginatedResponse<T> {
   items: T[];
   total: number;