Session billing and quota plug-in

Key features

• Fees are local reference values, not bills, and do not automatically block model calls.
• The host side is responsible for pricing and projection, and the browser side renders the interface from the projection calculated by the host.
• usage uses request/header to attribute step
• The installable root bundle exports both host and web client entries, so installation from GitHub does not rely on publishing two additional internal packages.
• The same model ID will be counted separately under different providers, such as deepseek/deepseek-v4-flash and openrouter/deepseek-v4-flash.