feat: landing page + logo, dev-mode banner + sync cap, non-blocking sync with progress splash

frontend/src/main.jsx

@@ -1,6 +1,7 @@
 import React from 'react';
 import ReactDOM from 'react-dom/client';
-import { BrowserRouter, Routes, Route } from 'react-router-dom';
+import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom';
+import Landing from './pages/Landing.jsx';
 import Dashboard from './pages/Dashboard.jsx';
 import Cleanup from './pages/Cleanup.jsx';
 import Unsubscribe from './pages/Unsubscribe.jsx';
@@ -11,11 +12,17 @@ ReactDOM.createRoot(document.getElementById('root')).render(
   <React.StrictMode>
     <BrowserRouter>
       <Routes>
-        <Route element={<Layout />}>
-          <Route path="/" element={<Dashboard />} />
-          <Route path="/cleanup" element={<Cleanup />} />
-          <Route path="/unsubscribe" element={<Unsubscribe />} />
+        {/* Public landing page */}
+        <Route path="/" element={<Landing />} />
+
+        {/* Authenticated app */}
+        <Route path="/app" element={<Layout />}>
+          <Route index element={<Dashboard />} />
+          <Route path="cleanup" element={<Cleanup />} />
+          <Route path="unsubscribe" element={<Unsubscribe />} />
         </Route>
+
+        <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>
     </BrowserRouter>
   </React.StrictMode>