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/package.json

@@ -50,6 +50,7 @@
     "react-day-picker": "^9.14.0",
     "react-dom": "^19.2.5",
     "react-is": "^19.2.5",
+    "react-router-dom": "^7.14.2",
     "react-use-measure": "^2.1.7",
     "recharts": "^3.8.1",
     "tailwind-merge": "^3.5.0",