feat(02-02): add preferences store with svelte-persisted-store

- Install svelte-persisted-store for localStorage persistence
- Create preferences store with lastEntryType and showCompleted
- Sticky type preference across sessions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/stores/preferences.svelte.ts

@@ -0,0 +1,6 @@
+import { persisted } from 'svelte-persisted-store';
+
+export const preferences = persisted('taskplaner-preferences', {
+	lastEntryType: 'thought' as 'task' | 'thought',
+	showCompleted: false
+});