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/i18n/locales/en.ts

@@ -31,16 +31,29 @@ const en = {
     forgetTitle: 'Forget this instance',
     form: {
       title: 'Connect to a backend',
+      registerTitle: 'Create an account',
       serverUrl: 'Server URL',
       username: 'Username',
       password: 'Password',
+      passwordHint: 'At least 8 characters.',
       submit: 'Connect',
       submitting: 'Connecting…',
+      registerSubmit: 'Create account',
+      registering: 'Creating account…',
+      noAccount: "Don't have an account?",
+      registerLink: 'Sign up',
+      haveAccount: 'Already have an account?',
+      signInLink: 'Sign in',
     },
     errors: {
-      unreachable: "Can't reach this server. Check the URL and that it's online.",
+      unreachable:
+        "Can't reach this server. Check the URL and that it's online.",
       badCredentials: 'Incorrect username or password.',
       generic: 'Sign-in failed. Please try again.',
+      usernameTaken: 'That username is already taken.',
+      passwordTooShort: 'Password must be at least 8 characters.',
+      registrationDisabled: 'Registration is disabled on this server.',
+      registerFailed: 'Could not create the account. Please try again.',
     },
   },
   library: {
@@ -211,6 +224,8 @@ const en = {
 export default en;
 
 type DeepString<T> = {
-  [K in keyof T]: T[K] extends Record<string, unknown> ? DeepString<T[K]> : string;
+  [K in keyof T]: T[K] extends Record<string, unknown>
+    ? DeepString<T[K]>
+    : string;
 };
 export type Translations = DeepString<typeof en>;