fix: replace optimistic workspace removal with destroying state indicator (Refs: beans-9s9h)

[hmans]

Summary

• When destroying a workspace, the optimistic removal caused a visual flash: the item disappeared from the sidebar, reappeared when the subscription delivered the still-existing worktree, then disappeared again once the backend finished cleanup.
• Instead of eagerly removing, workspaces being destroyed now render at 30% opacity with interactions disabled, giving clear visual feedback while the backend processes the removal.
• The subscription handler automatically cleans up the destroying state once the worktree is gone from the backend's list.

Test plan

• Create a workspace, then destroy it — verify it fades to low opacity and stays visible until fully removed (no flash/reappear)
• Verify the destroy button in WorkspaceView is disabled and shows "Destroying..." tooltip during destruction
• Verify that if the mutation fails, the workspace returns to normal opacity