fix: address PR review comments

- Wrap readFile in try/catch, return 404 for missing attachment files
- Remove unused CollaboratorPopover import from NoteCard
- Make darkMode a proper $state rune for reactivity (add svelte-runes
  polyfill for Vitest so $state works in unit tests)
- Add error toasts for failed share link toggle (non-2xx responses)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vitest.config.ts

@@ -11,6 +11,7 @@ export default defineConfig({
 	test: {
 		include: ['src/**/*.test.ts'],
 		environment: 'node',
+		setupFiles: ['src/lib/__mocks__/svelte-runes.ts'],
 		coverage: {
 			provider: 'v8',
 			reporter: ['text', 'html', 'lcov', 'json-summary'],