T5/Phase 3: SPA shell + navigation

- TanStack Router with type-safe file-based routes
- Sidebar: collapsible, workspace switcher, 12 nav links, user menu
- Topbar: search, quick-add, notifications, user avatar
- cmdk command palette: nav + create + search + @mentions
- Keyboard shortcuts: Cmd+K palette, g+t/h/p/n/c/g/s navigation, ? help
- Theme: dark default, accent picker (slate/blue/green/purple/orange), persisted to localStorage
- Auth: login -> /, logout -> /login, 401 redirect
- Placeholder pages for all 13 nav routes (real in T6/T7)
- Typed API client with useApiQuery/useApiMutation hooks

apps/web/src/routes/_app.tsx

@@ -0,0 +1,35 @@
+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();
+
+  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-4 md:p-6"
+          tabIndex={-1}
+        >
+          <Outlet />
+        </main>
+      </div>
+      <CommandPalette />
+      <ShortcutsHelp />
+    </div>
+  );
+}
+
+export const Route = createRoute({
+  getParentRoute: () => rootRoute,
+  id: "_app",
+  component: AppLayout,
+});