feat(ui): Create content pattern components - DataTable, FilterBar, EmptyState, LoadingState

- Add LoadingState with table, card, list, and text skeleton patterns
- Add EmptyState with customizable icon, title, description, and action slot
- Add FilterBar with search input, clear button, and custom filter slot
- Add DataTable with TanStack Table integration, sorting, and row click
- Create barrel export index.ts for common components
- Install tanstack-table-8-svelte-5 for Svelte 5 compatibility
- Sync auth spec with authenticated user redirect requirements
- Archive p03-dashboard-enhancement

Refs: openspec/changes/p04-content-patterns
Closes: p04-content-patterns

frontend/tests/component/EmptyState.spec.ts

@@ -0,0 +1,8 @@
+import { describe, it, expect } from 'vitest';
+import EmptyState from '$lib/components/common/EmptyState.svelte';
+
+describe('EmptyState', () => {
+	it('exports a component module', () => {
+		expect(EmptyState).toBeDefined();
+	});
+});