refactor: move shared components to lib/shared/ sub-packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/hooks/useUnsavedWarning.svelte.test.ts

@@ -14,7 +14,7 @@ vi.mock('$app/navigation', () => ({
 	goto: mockGoto
 }));
 
-const { createUnsavedWarning } = await import('../useUnsavedWarning.svelte');
+const { createUnsavedWarning } = await import('./useUnsavedWarning.svelte');
 
 function simulateNavigate(href: string | null = '/somewhere') {
 	const cancel = vi.fn();