Refactor board workspace lifecycle and harden delete confirmations

[devmobasa]

Summary

• Refactor board workspace behavior into focused color, identity, and operation modules.
• Add generation-backed board/page delete confirmations so stale confirmations are rejected after topology changes.
• Clear pending delete confirmations and actionable delete toasts when applying session snapshots.
• Fix page undo restoration after confirming a delete while another board is active.
• Make actionable toast clicks target-stable across press/release so undo/delete clicks do not also start canvas input or transfer to a replacement toast.
• Clear stale toast bounds when replacing toasts so old geometry cannot activate a new toast.