Fix bugs, migrate favorites store to Composition API, DRY state loader, add page tests, replace CSS vars with kit classes

Move unassign into a dedicated confirm dialog and fix icon ...

- Revert useAreaAssign composable back to pure assign logic; remove
  submitUnassignCore and unassign state that leaked into the assign modal.
- DeviceDetailPage, ScriptDetailPage, AreaDetailPage:
  * Remove the Unassign button from the assign modal footer.
  * Add a separate GnModal confirm dialog for unassign (like Remove).
  * Dropdown action now opens the confirm dialog instead of firing
    immediately.
  * Replace non-existent ph-map-pin-slash icon with ph-x-circle.

Add unassign action to area, device and script detail pages ...

- useAreaAssign composable: add submitUnassignCore and loading/error states.
- DeviceDetailPage: add 'Unassign from area' button inside assign modal and
  in the actions dropdown when device has an area_id.
- ScriptDetailPage: same for scripts (non-scope) with area_id.
- AreaDetailPage: add 'Unassign from parent' in assign modal and dropdown
  when area has a parent_id > 0.

Fix area assignment button on detail pages ...

- Rename submitAssign -> submitAssignCore in useAreaAssign composable
  to avoid name collision with local page handlers.
- Rename local page handlers -> handleAssignSubmit on Area, Device,
  and Script detail pages so event bindings call the wrapper correctly.
- Fix AppEmptyState.vue to forward #action slot into GnEmptyState's
  #actions slot so the Assign button renders for unassigned items.

Phase 5: Extract reusable useAsyncRequest composable for stores ...

- Add src/composables/useAsyncRequest.js to manage isLoading/error/AbortController
- Convert Pinia stores to Setup Stores: areas, devices, scripts, scanning
- Use useAsyncRequest for all simple list/detail loaders
- Fix API modules to forward options/signal (areas.devices, areas.scripts,
  devices.detail, scripts.scopeCode) — previously AbortControllers were
  created but signals were never passed to fetch
- Keep batch loadDeviceStates and reboot/runScript as custom logic
- Use immutable Set updates for reactive rebootingIds/runningAliases
- Update modules.spec.js assertions for new optional arguments
- Add 9 unit tests for useAsyncRequest

All 139 tests pass.

Add toast notifications for mutations (Phase 3) ...

- Wrap RouterView in GnToastProvider in App.vue
- Replace inline rebootError alert in DeviceDetailPage with toast
- Replace inline resultAlert in ScriptDetailPage and ScriptsActionsPage with toast
- Add success toasts after successful edit, assign, remove, rename, create, setup
- Keep inline GnAlert inside modals for form-level errors

Add global error handling (Phase 2) ...

- Create useGlobalErrorHandler composable that wires up three
  global error-catching surfaces:
    1. app.config.errorHandler — Vue runtime errors (render,
       watchers, lifecycle hooks, event handlers).
    2. window 'unhandledrejection' — rejected promises without catch.
    3. window 'error' — synchronous JS runtime errors.
- Create AppErrorBoundary component wrapping RouterView to catch
  Vue errors in child components and show fallback UI (AppErrorState
  with Retry) instead of a blank screen.
- Apply the boundary in App.vue so every routed page is protected.

All 126 Vitest tests pass.

Vue client: add shared area assignment to scripts and devices ...

- Add useAreaAssign composable for reusable area assignment logic
- Add AreaBadgeLink and AreaAssignSection shared components
- Integrate area assignment into ScriptDetailPage (actions/regular)
- Integrate area assignment into DeviceDetailPage
- Update scriptsStore with assignToArea / unassignFromArea + reactivity fixes
- Update API modules: scripts.placeInArea, devices.assignToArea
- Update list views: show area badges in actions/regular cards and tables
- Add tests and mock handlers for script area operations
- Update AppShell nav and routes for detail pages
- Add prism-theme.css for code highlighting
- 125 tests passing, build green