Unify toast notifications and hide redundant Todo comment action

[jadon7]

Changes

• Replace the global message and undo notifications with one capsule toast stack, independently implemented from the approved Kobra visual reference. Preserve undo actions and keyboard shortcuts; support hover/focus pause, stacked expansion, and drag dismissal.
• Hide the comment composer’s “Change status to Todo” option when the issue is already Todo. Keep the option and comment-triggered status change for other states.

Verification

• npm run typecheck and npm run build:web passed.
• Verified in a real browser against an isolated local database: copy ID/link, toast stacking and expansion, auto-dismiss, drag dismissal, archive and undo (click/keyboard), Todo option absence, and non-Todo comment submission changing status to Todo.
• Agent review passed after correcting focus cleanup and pointer movement across expanded stack gaps. No new tests or dependencies.

Tracks LOCAL-450 and LOCAL-451. Kobra source is not included; the user approved independent implementation of the reference appearance and interaction.