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

@@ -39,22 +39,20 @@ 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" }}>
-          <Routes>
-            <Route path="/" element={<Write />} />
-            <Route path="/characters" element={<Characters />} />
-            <Route path="/plot" element={<Plot />} />
-            <Route path="/whiteboard" element={<Whiteboard />} />
-            <Route path="/ai" element={<AI />} />
-            <Route path="/projects" element={<Projects />} />
-            <Route path="/settings" element={<Settings />} />
-            <Route path="/admin" element={<Admin />} />
-            <Route path="*" element={<Write />} />
-          </Routes>
-        </main>
-      </div>
+      <Topbar onOpenPalette={() => setPaletteOpen(true)} />
+      <main className="ps-main">
+        <Routes>
+          <Route path="/" element={<Write />} />
+          <Route path="/characters" element={<Characters />} />
+          <Route path="/plot" element={<Plot />} />
+          <Route path="/whiteboard" element={<Whiteboard />} />
+          <Route path="/ai" element={<AI />} />
+          <Route path="/projects" element={<Projects />} />
+          <Route path="/settings" element={<Settings />} />
+          <Route path="/admin" element={<Admin />} />
+          <Route path="*" element={<Write />} />
+        </Routes>
+      </main>
 
       <CommandPalette open={paletteOpen} onClose={() => setPaletteOpen(false)} nav={NAV} />
       <ToastViewport />