Backup/restore and note pinning

Added two small QoL features to FlowKit today, both addressing gaps I noticed after using it for a while.

Backup & Restore. FlowKit only ever stored data in localStorage, which means a cleared cache or a new browser wipes out every task and note with no way back. There’s now a 💾 button in the sidebar (and a command palette entry) that opens a modal with two options: Export downloads a timestamped JSON file with all tasks, notes, and recent colors; Import reads a JSON file back in and merges it non-destructively (skips anything with an ID you already have, so it’s safe to import the same file twice).

Note pinning. Notes were a flat, most-recently-created list, so anything you wanted to keep handy just scrolled away. Each note now has a 📌 toggle, both on its row in the sidebar list and in the editor topbar. Pinned notes always sort to the top, so reference notes stay put instead of getting buried under whatever you jotted last.

Both features reuse the existing dark theme and modal/toast components rather than introducing new UI patterns, so they should feel native to the rest of the app.