ui: add company-aware not found handling

ui/src/App.tsx

@@ -28,6 +28,7 @@ import { NewAgent } from "./pages/NewAgent";
 import { AuthPage } from "./pages/Auth";
 import { BoardClaimPage } from "./pages/BoardClaim";
 import { InviteLandingPage } from "./pages/InviteLanding";
+import { NotFoundPage } from "./pages/NotFound";
 import { queryKeys } from "./lib/queryKeys";
 import { useCompany } from "./context/CompanyContext";
 import { useDialog } from "./context/DialogContext";
@@ -141,6 +142,7 @@ function boardRoutes() {
       <Route path="inbox/new" element={<Inbox />} />
       <Route path="inbox/all" element={<Inbox />} />
       <Route path="design-guide" element={<DesignGuide />} />
+      <Route path="*" element={<NotFoundPage scope="board" />} />
     </>
   );
 }
@@ -240,6 +242,7 @@ export function App() {
           <Route path=":companyPrefix" element={<Layout />}>
             {boardRoutes()}
           </Route>
+          <Route path="*" element={<NotFoundPage scope="global" />} />
         </Route>
       </Routes>
       <OnboardingWizard />