Refactor frontend to hook-based architecture

Extract logic from god components into custom hooks (useAuthCheck,
useConversations, useChat, usePresignedUrl, useAdminUsers, useOIDCAuth).
Eliminate unnecessary useEffects per React guidelines — scroll is now
imperative, isAdmin comes from useAuthCheck instead of a separate fetch.
ConversationList becomes a pure presentational component. Wrap bubble
components in React.memo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

raggr-frontend/src/hooks/useOIDCAuth.ts

@@ -0,0 +1,59 @@
+import { useState, useEffect } from "react";
+import { userService } from "../api/userService";
+import { oidcService } from "../api/oidcService";
+
+type UseOIDCAuthOptions = {
+  setAuthenticated: (isAuth: boolean) => void;
+};
+
+export function useOIDCAuth({ setAuthenticated }: UseOIDCAuthOptions) {
+  const [isChecking, setIsChecking] = useState(true);
+  const [isLoggingIn, setIsLoggingIn] = useState(false);
+  const [error, setError] = useState("");
+
+  useEffect(() => {
+    const initAuth = async () => {
+      const callbackParams = oidcService.getCallbackParamsFromURL();
+      if (callbackParams) {
+        try {
+          setIsLoggingIn(true);
+          const result = await oidcService.handleCallback(
+            callbackParams.code,
+            callbackParams.state,
+          );
+          localStorage.setItem("access_token", result.access_token);
+          localStorage.setItem("refresh_token", result.refresh_token);
+          oidcService.clearCallbackParams();
+          setAuthenticated(true);
+          setIsChecking(false);
+          return;
+        } catch (err) {
+          console.error("OIDC callback error:", err);
+          setError("Login failed. Please try again.");
+          oidcService.clearCallbackParams();
+          setIsLoggingIn(false);
+          setIsChecking(false);
+          return;
+        }
+      }
+      const isValid = await userService.validateToken();
+      if (isValid) setAuthenticated(true);
+      setIsChecking(false);
+    };
+    initAuth();
+  }, [setAuthenticated]);
+
+  const handleLogin = async () => {
+    try {
+      setIsLoggingIn(true);
+      setError("");
+      const authUrl = await oidcService.initiateLogin();
+      window.location.href = authUrl;
+    } catch {
+      setError("Failed to initiate login. Please try again.");
+      setIsLoggingIn(false);
+    }
+  };
+
+  return { isChecking, isLoggingIn, error, handleLogin };
+}