Toast-based feedback with undo for every mutation; confirm or undo for Archive

Part of PAR-357. Needs the Toast primitive from the design-system task.

Problem

• Feedback text moves the layout. Feedback after actions is inline text that shifts it:
  • "Saving…" / "Saved." under forms (DS/Form/Form.tsx:133-137)
  • "Comment added." under the composer
  • "Saving…" / "Saved." StateBlocks above the grid after a drag
  • "Updating results…" under filters
  • an "Undo deleted view" button that appears inline
• The new-issue banner "PAR-123 created" appears at the top of the form.
• Archive has no safety net. It runs on one click from the issue header with no confirmation or undo (issues/[ref]/page.tsx:130-136).

Do

• Add a global toaster in the bottom-left: success with an optional action (View, Undo), error with Retry, and a loading → success promise toast for slow actions.
• Give these an Undo:
  • archive
  • delete view
  • bulk edits
  • status changes made with keyboard shortcuts
  • removing a label
• Remove every inline Saving/Saved message; pending state lives on the control itself (spinner in the pill or button).
• Show a "Saved" check in the issue header only for autosaving description edits.