test: dedup sonner mocks and close Toolbar seam-mock gaps

- Replace 10 inline sonner toast mocks with the shared mockSonner() builder.
- Toolbar: add tests for the store-driven undo/redo disabled state and the
  unsaved-changes dot — behaviour the full-store mock previously left
  unasserted.

ha-relevant: yes

frontend/src/components/modals/__tests__/ThemeModal.test.tsx

@@ -5,7 +5,7 @@ import { useThemeStore } from '@/stores/themeStore'
 import { useCanvasStore } from '@/stores/canvasStore'
 import { THEME_ORDER } from '@/utils/themes'
 
-vi.mock('sonner', () => ({ toast: { success: vi.fn(), error: vi.fn(), info: vi.fn() } }))
+vi.mock('sonner', async () => (await import('@/test/mocks')).mockSonner())
 import { toast } from 'sonner'
 
 describe('ThemeModal', () => {