refactor(settings): replace modal with nested full-page route

Settings sections are now URL-addressable at /settings/:sectionId, rendered
nested inside EditorLayout alongside the stack sidebar. Browser back/forward
navigates between sections. Deep links (e.g. /settings/cloud-backup) load
the section directly on hard reload.

- Add react-router-dom v7; BrowserRouter wraps the full app tree
- New SettingsPage (scroll memory, Cmd+K palette), SettingsSidebar (NavLink
  active styling, back-arrow), SectionGate (visibility + tier lock card)
- Rename SectionId 'appstore' to 'app-store' so slug === SectionId
- Decouple SystemSection, DeveloperSection, AppStoreSection from modal-
  passed props; each fetches its own data on mount
- Replace onLabelsChanged prop chain with SENCHO_LABELS_CHANGED window event
- Drop onOpenSettings prop from UserProfileDropdown, HomeDashboard,
  ConfigurationStatus; each calls useNavigate directly
- Delete SettingsModal.tsx

frontend/src/App.tsx

@@ -1,3 +1,4 @@
+import { BrowserRouter } from 'react-router-dom';
 import { AuthProvider, useAuth } from './context/AuthContext';
 import { NodeProvider } from './context/NodeContext';
 import { LicenseProvider } from './context/LicenseContext';
@@ -44,13 +45,15 @@ import { ToastContainer } from './components/ui/toast';
 
 function App() {
   return (
-    <AuthProvider>
-      <DeployFeedbackProvider>
-        <AppContent />
-        <DeployFeedbackPortal />
-      </DeployFeedbackProvider>
-      <ToastContainer />
-    </AuthProvider>
+    <BrowserRouter>
+      <AuthProvider>
+        <DeployFeedbackProvider>
+          <AppContent />
+          <DeployFeedbackPortal />
+        </DeployFeedbackProvider>
+        <ToastContainer />
+      </AuthProvider>
+    </BrowserRouter>
   );
 }