v0.2.1

Bump version to 0.2.1

Docs-only release. v0.2.0 shipped the new account + history commands
(balance, price, history, show, delete, top-up, open) and dynamic --help
URL. This patch propagates the z-image/turbo example swap that landed
in 816dc67 into the published npm package — `wavespeed --help`,
`wavespeed login`'s "Try it:" footer, and `wavespeed schema --help`
now suggest `wavespeed-ai/z-image/turbo` (~5s runs) instead of
`google/nano-banana-2/text-to-image` (40-90s). Nano-banana-2 stays as
the production-quality recommendation in the README.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>