Reapply "Add Turnstile captcha to login"

This reverts commit a7c732e139.

src/vite-env.d.ts

@@ -0,0 +1,10 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_API_URL?: string;
+  readonly VITE_TURNSTILE_SITE_KEY?: string;
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv;
+}