Adds a drafts system allowing users to save input content for later use.
- Rust backend: list, save, delete, delete-all commands backed by hikari-drafts.json
- Svelte store wrapping all four Tauri commands with timestamp formatting
- DraftPanel overlay with insert, per-item delete confirmation, and delete-all
- InputBar: Drafts button in control row, floppy-disk Save as Draft icon button
