Fix auth login redirect loop

frontend/components/auth/LoginClient.tsx

@@ -169,20 +169,6 @@ export function LoginClient({ nextPath, initialError, initialMode }: LoginClient
     }
   };
 
-  useEffect(() => {
-    if (!supabaseReady) return;
-    const run = async () => {
-      const supabase = getSupabaseBrowserClient();
-      const {
-        data: { session },
-      } = await supabase.auth.getSession();
-      if (session?.user) {
-        router.replace(nextPath);
-      }
-    };
-    void run();
-  }, [nextPath, router, supabaseReady]);
-
   const onGoogleSignIn = async () => {
     setErrorText("");
     setInfoText("");

frontend/components/auth/__tests__/loginRedirectLoop.test.ts

@@ -0,0 +1,28 @@
+import fs from "node:fs";
+import path from "node:path";
+
+function assert(condition: unknown, message: string) {
+  if (!condition) throw new Error(message);
+}
+
+export function runTests() {
+  const source = fs.readFileSync(
+    path.join(process.cwd(), "components", "auth", "LoginClient.tsx"),
+    "utf8",
+  ).replace(/\r\n/g, "\n");
+
+  const start = source.indexOf("const onResetPassword");
+  const end = source.indexOf("const onGoogleSignIn");
+
+  assert(
+    start >= 0 && end > start,
+    "login page pre-submit auth initialization block must be detectable by this regression test",
+  );
+
+  const loginMountEffect = source.slice(start, end);
+
+  assert(
+    !loginMountEffect.includes("router.replace(nextPath)"),
+    "login page must not auto-return to nextPath from a cached Supabase session before the user submits auth",
+  );
+}