Make pasting reliable and turn items into checkboxes

Pasting still failed because the panel is a nonactivating panel: it can
be key while the app is not active, and menu key equivalents are not
delivered reliably in that state. Cmd+V is now dispatched to the
responder chain directly, and the app no longer replaces SwiftUI's main
menu — it only adds Hide Panel to it, so the Edit items that AppKit
routes text shortcuts through stay intact.

Rows also gained a checkbox so a workspace works as a checklist without
turning the row into a button, which would swallow the checkbox's clicks.

README.md

@@ -4,6 +4,8 @@ A native macOS app for keeping prompts and photos, in the spirit of
 [Copper](https://shadcn.com/copper) by shadcn: a floating panel that sits next
 to your work. Click anything to copy it back to your clipboard.
 
+- **Checklists by default** — every item has a checkbox, so a workspace doubles
+  as a to-do list; done items stay in place, dimmed and struck through.
 - **Click to copy** — text notes go back as text, photos go back as images.
 - **Capture from anywhere** — select text in any app and tap **Left Shift twice**.
 - **Photos** — paste (Cmd+V), drag & drop, or add via the photo button.