fix: move session check after all hooks to fix blank page on login

Early conditional return before useEffect/useMemo violated React Rules
of Hooks, causing error #300 (rendered more hooks than previous render)
after login.

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

src/App.tsx

@@ -45,14 +45,6 @@ export default function App() {
     setSession(null);
   };
 
-  if (!session) {
-    return <LoginPage onLogin={() => {
-      const stored = getStoredSession();
-      console.log('[App] onLogin, stored session:', stored ? 'found' : 'null');
-      setSession(stored);
-    }} />;
-  }
-
   useEffect(() => {
     const loadDefaultData = async () => {
       // In dev: Vite proxy handles /dropbox-file (see vite.config.ts)
@@ -157,8 +149,8 @@ export default function App() {
       }
     };
 
-    loadDefaultData();
-  }, []);
+    if (session) loadDefaultData();
+  }, [session]);
 
   const handleFileUpload = (e: React.ChangeEvent<HTMLInputElement>) => {
     const file = e.target.files?.[0];
@@ -374,6 +366,14 @@ export default function App() {
     };
   }, [appState.data]);
 
+  if (!session) {
+    return <LoginPage onLogin={() => {
+      const stored = getStoredSession();
+      console.log('[App] onLogin, stored session:', stored ? 'found' : 'null');
+      setSession(stored);
+    }} />;
+  }
+
   return (
     <div className="h-screen bg-[#040d1a] text-slate-200 flex flex-col font-sans overflow-hidden">
       <TopBar