feat(ui): delete a checkpoint from the chat, beside Fork (#2804)

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*🤖 written by Claude (start)*

A checkpoint's line now carries `Delete` beside `Fork`, behind a
confirmation. Deleting releases the snapshot that checkpoint pins; chats
already forked from it keep working.

UI only — `CheckpointService/DeleteCheckpoint` already existed, so this
wires it through the api layer and the fixture backend. Mock checkpoints
move to `sessionStorage`, so a deletion in mock mode survives a reload
the way the controller's does.

## Testing

1. `ENABLE_MOCK_UI=true yarn dev`, open a conversation, and save two
checkpoints a turn apart.
2. Press `Delete` on the earlier line and confirm. That line goes; the
other stays.
3. Reload. The deleted line does not come back.

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*🤖 written by Claude (end)*

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Signed-off-by: Nicholas Bucher <behappy54321@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>