Wire optimistic-locking 409 into the client

[shreydevkar]

The server supports expected_versions + 409 Conflict on /grid/cell and /grid/range. The frontend doesn't send it or handle it yet. Wire a "someone else edited that cell — reload?" toast when a local write races a remote one.

Acceptance criteria:

• persistSingleCell / persistRange send expected_versions from the last known grid snapshot.
• 409 response renders a non-modal toast with Reload / Dismiss actions.
• Reload refetches the grid without clobbering in-progress edits on other cells.