feat(levelcode): add "Continue with GitHub" to the Cloud login

src/levelcode/pages/ProviderSignIn.tsx

@@ -1,29 +1,55 @@
+import type { ReactNode } from "react";
 import { API_BASE } from "../api";
 
-// "Continue with Google" for the LevelCode Cloud login.
+// Social sign-in for the LevelCode Cloud login: Google + GitHub.
 //
-// A real <a> (top-level GET), NOT an api() fetch and NOT the GIS id_token button: Rails' oauth_start
-// 302s to Google and relies on the session cookie it sets to carry the CSRF `state`, neither of which
-// survives an XHR. As a link it keeps native semantics too — keyboard activation, Cmd/Ctrl-click to
-// open in a new tab, and it works without JS.
+// Each is a real <a> (top-level GET), NOT an api() fetch and NOT a GIS-style token button: Rails'
+// oauth_start 302s to the provider and relies on the session cookie it sets to carry the CSRF `state`,
+// neither of which survives an XHR. As links they keep native semantics too — keyboard, Cmd/Ctrl-click
+// to open in a new tab, works without JS.
 //
-// The editor's redirect_uri + code_challenge are a PKCE PAIR: forwarded only when BOTH are present, so a
-// partial (unbound) handoff never reaches the backend. Absent — a plain web login — oauth_start just
-// opens a web session. The backend already accepts `google` here; GitHub would come "free" the same way.
+// The editor's redirect_uri + code_challenge are a PKCE PAIR, forwarded only when BOTH are present (see
+// oauthStartUrl) so a partial handoff never reaches the backend. Both providers share the exact same
+// oauth_start/oauth_callback path — the callback dispatches on the stashed provider.
 export default function ProviderSignIn({
   redirectUri,
   codeChallenge,
 }: {
   redirectUri?: string;
   codeChallenge?: string;
+}) {
+  return (
+    <div className="flex flex-col gap-3">
+      <ProviderLink provider="google" label="Continue with Google" redirectUri={redirectUri} codeChallenge={codeChallenge}>
+        <GoogleG />
+      </ProviderLink>
+      <ProviderLink provider="github" label="Continue with GitHub" redirectUri={redirectUri} codeChallenge={codeChallenge}>
+        <GitHubMark />
+      </ProviderLink>
+    </div>
+  );
+}
+
+function ProviderLink({
+  provider,
+  label,
+  redirectUri,
+  codeChallenge,
+  children,
+}: {
+  provider: string;
+  label: string;
+  redirectUri?: string;
+  codeChallenge?: string;
+  children: ReactNode;
 }) {
   return (
     <a
-      href={oauthStartUrl("google", redirectUri, codeChallenge)}
+      href={oauthStartUrl(provider, redirectUri, codeChallenge)}
       className="flex w-full items-center justify-center gap-2.5 rounded-full border border-rule bg-card px-6 py-3 text-[15px] font-medium text-ink transition-colors hover:border-ink"
     >
-      <GoogleG />
-      Continue with Google
+      {children}
+      {label}
     </a>
   );
 }
@@ -62,3 +88,11 @@ function GoogleG() {
     </svg>
   );
 }
+
+function GitHubMark() {
+  return (
+    <svg width="18" height="18" viewBox="0 0 24 24" fill="currentColor" aria-hidden="true" className="shrink-0">
+      <path d="M12 .297c-6.63 0-12 5.373-12 12 0 5.303 3.438 9.8 8.205 11.385.6.113.82-.258.82-.577 0-.285-.01-1.04-.015-2.04-3.338.724-4.042-1.61-4.042-1.61C4.422 18.07 3.633 17.7 3.633 17.7c-1.087-.744.084-.729.084-.729 1.205.084 1.838 1.236 1.838 1.236 1.07 1.835 2.809 1.305 3.495.998.108-.776.417-1.305.76-1.605-2.665-.3-5.466-1.332-5.466-5.93 0-1.31.465-2.38 1.235-3.22-.135-.303-.54-1.523.105-3.176 0 0 1.005-.322 3.3 1.23.96-.267 1.98-.399 3-.405 1.02.006 2.04.138 3 .405 2.28-1.552 3.285-1.23 3.285-1.23.645 1.653.24 2.873.12 3.176.765.84 1.23 1.91 1.23 3.22 0 4.61-2.805 5.625-5.475 5.92.42.36.81 1.096.81 2.22 0 1.606-.015 2.896-.015 3.286 0 .315.21.69.825.57C20.565 22.092 24 17.592 24 12.297c0-6.627-5.373-12-12-12" />
+    </svg>
+  );
+}