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/config/env.ts

@@ -20,3 +20,22 @@ function runtimeApiBaseUrl(): string | undefined {
 
 export const DEFAULT_API_BASE_URL =
   runtimeApiBaseUrl() ?? import.meta.env.PUBLIC_API_BASE_URL ?? '/api/v1';
+
+/**
+ * Whether the public sign-up UI is shown. Same precedence as the base URL:
+ * runtime operator config (injected into `window.__APP_CONFIG__` at container
+ * start) wins over the build-time `PUBLIC_ENABLE_REGISTRATION` env, which
+ * defaults to enabled. This only gates the *UI*; the backend independently
+ * enforces `ALLOW_REGISTRATION` and is the real authority.
+ */
+function parseFlag(value: string | undefined): boolean | undefined {
+  if (value == null || value === '') return undefined;
+  return value !== 'false' && value !== '0';
+}
+
+export const REGISTRATION_ENABLED: boolean =
+  (typeof window !== 'undefined'
+    ? window.__APP_CONFIG__?.enableRegistration
+    : undefined) ??
+  parseFlag(import.meta.env.PUBLIC_ENABLE_REGISTRATION) ??
+  true;