Undo for triage actions + more consistent toast notifications

Two related improvements around triage feedback: an undo for the last action, and a cleaner, more consistent toast/notification system that surfaces that undo.

Undo

It's very easy to mis-click or accidentally trigger a bulk triage action with no quick way to reverse it. Add an undo for the last triage action (approve / flag / trash), for both single and bulk actions:

• Keyboard: Ctrl/Cmd+Z to undo the last action.
• Toast link: an [undo] link in the confirmation toast, e.g. "5 items marked as trash [undo]" or "Item approved [undo]".

Toast notifications

The current toasts need work — they stack up and feel inconsistent. Improvements requested:

• Don't stack: repeated actions should collapse/replace rather than piling multiple toasts on top of each other.
• Consistent format: a single, uniform style and wording across all triage actions (approve / flag / trash, single and bulk).
• Built-in undo: every action toast should carry the [undo] link described above.

Reported via support: a user repeatedly trashed and then accidentally approved the wrong batch and wanted a one-click way back, alongside tidier notifications.