feat: prioritize game iframe, gated register terms and polished register page

Fetch the Nitro client iframe with high priority so the browser starts the
game document before competing resources, and replace the bare /client
spinner with a branded boot screen. Preconnect and eager loading were already
in place; typing support for the iframe fetchPriority prop is added in a
React type augmentation.

Rework the register terms block into a single clickable accept control with
a custom check state, error shake and inline hint, and dim the submit button
until the terms are accepted. The register page gets labeled sections
(account details / credentials), a corrected banner overlay, translated
show/hide toggles and a captcha slot that reserves height to avoid layout
shifts. English is the source of truth; other locales fall back to it.

src/types/react-dom.d.ts

@@ -0,0 +1,13 @@
+import type { HTMLAttributes } from "react";
+
+declare module "react" {
+	interface IframeHTMLAttributes<T> extends HTMLAttributes<T> {
+		/**
+		 * Browser support for `fetchpriority` on iframes exists (Chrome 126+,
+		 * Safari 17.5+, Firefox 129+); React forwards the `fetchPriority` prop
+		 * to a `fetchpriority` attribute so the game document is fetched with
+		 * high priority instead of competing with other resources.
+		 */
+		fetchPriority?: "high" | "low" | "auto";
+	}
+}