fix(web): restore _app.tsx layout route deleted in 2175c24 (blank-screen fix)

Commit 2175c24 deleted apps/web/src/routes/_app.tsx alongside __root.tsx.
Only __root was restored (28b2cbd). With _app.tsx missing, routeTree.ts's
import './routes/_app' resolved to the _app/ directory index (dashboard
route), making the route its own child — TanStack buildRouteTree throws
'Invariant failed' at module init and React never mounts (blank page, no
console error). Restores the exact file: _app pathless layout with
Sidebar/Topbar/Outlet.

apps/web/src/routes/_app.tsx

@@ -0,0 +1,52 @@
+import { useEffect } from "react";
+import { createRoute, Outlet } from "@tanstack/react-router";
+import { Route as rootRoute } from "./__root";
+import { Sidebar } from "@/components/shell/sidebar";
+import { Topbar } from "@/components/shell/topbar";
+import { CommandPalette } from "@/components/shell/command-palette";
+import { ShortcutsHelp } from "@/components/shell/shortcuts-help";
+import { useKeyboardShortcuts } from "@/hooks/use-keyboard-shortcuts";
+
+function AppLayout() {
+  useKeyboardShortcuts();
+
+  // Apply persisted appearance preferences (density, reduced motion, font size)
+  // right after the first paint. The settings page updates these live while
+  // open; this covers reloads where the settings page was never visited.
+  useEffect(() => {
+    const root = document.documentElement;
+    root.classList.remove("density-compact", "density-spacious", "reduce-motion");
+    const density = localStorage.getItem("density");
+    if (density === "compact") root.classList.add("density-compact");
+    if (density === "spacious") root.classList.add("density-spacious");
+    if (localStorage.getItem("reduced-motion") === "true") root.classList.add("reduce-motion");
+    const fontSize = localStorage.getItem("font-size");
+    if (fontSize === "large") root.style.fontSize = "18px";
+    else if (fontSize === "small") root.style.fontSize = "13px";
+    else root.style.fontSize = "16px";
+  }, []);
+
+  return (
+    <div className="flex min-h-screen">
+      <Sidebar />
+      <div className="flex flex-1 flex-col">
+        <Topbar />
+        <main
+          id="main-content"
+          className="flex-1 overflow-auto p-[calc(1rem*var(--density-scale))] md:p-[calc(1.5rem*var(--density-scale))]"
+          tabIndex={-1}
+        >
+          <Outlet />
+        </main>
+      </div>
+      <CommandPalette />
+      <ShortcutsHelp />
+    </div>
+  );
+}
+
+export const Route = createRoute({
+  getParentRoute: () => rootRoute,
+  id: "_app",
+  component: AppLayout,
+});