test(dashboard): exercise the EnrichmentBlock skeleton branch

Hoists the $navigating store into a shared __mocks__ module so tests can
drive it through real transitions. Adds two specs covering (a) skeleton
visible while $navigating && topDocs empty and (b) skeleton hidden when
topDocs is non-empty. Also sets aria-busy="true" on the skeleton so
screen readers announce the loading state (Leonie's a11y suggestion).

Addresses Sara's and Felix's review concern that the skeleton branch was
dead code in the test world.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/__mocks__/navigatingStore.ts

@@ -0,0 +1,3 @@
+import { writable } from 'svelte/store';
+
+export const navigatingStore = writable<unknown | null>(null);