feat: add saved drafts feature

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

vitest.setup.ts

@@ -73,6 +73,8 @@ vi.mock("@tauri-apps/api/core", () => ({
         return Promise.resolve([]);
       case "list_clipboard_entries":
         return Promise.resolve([]);
+      case "list_drafts":
+        return Promise.resolve([]);
       case "cleanup_temp_files":
         return Promise.resolve();
       case "validate_directory":