fix(web): restore main area height in app shell grid

The App.tsx wrapped Topbar + main in a flex column div with height:100%,
which the .ps-app CSS grid auto-placed into the 48px topbar cell, leaving
the main grid area with zero height and a blank write view. Removed the
wrapper so Topbar (grid-area: topbar) and main (grid-area: main) are direct
grid children and receive their proper grid cells.

apps/web/src/App.tsx

@@ -39,9 +39,8 @@ export default function App() {
   return (
     <div className="ps-app">
       <Sidebar nav={NAV} />
-      <div style={{ display: "flex", flexDirection: "column", height: "100%" }}>
       <Topbar onOpenPalette={() => setPaletteOpen(true)} />
-        <main className="ps-main" style={{ flex: 1, overflow: "hidden" }}>
+      <main className="ps-main">
         <Routes>
           <Route path="/" element={<Write />} />
           <Route path="/characters" element={<Characters />} />
@@ -54,7 +53,6 @@ export default function App() {
           <Route path="*" element={<Write />} />
         </Routes>
       </main>
-      </div>
 
       <CommandPalette open={paletteOpen} onClose={() => setPaletteOpen(false)} nav={NAV} />
       <ToastViewport />