AI Credits improvements — multi-buyer and channel list

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Summary

Improve the AI Credits widget so users can manage multiple buyers and see the channels they have opened.

Improvements

Multiple buyers

Today a payer wallet maps to a single deterministic buyer. Users should be able to:

• Create and switch between multiple buyer identities from one payer wallet
• Paste private key for buyer key generation
• Recover a known buyer after refresh (deterministic derivation per buyer index/label)
• See Manage actions scoped to the selected buyer (balance, consent, withdraw, API setup)
• Filter History by buyer

Opened channels

Today closing a channel requires pasting a channel ID by hand. Users should be able to:

• See a list of channels they have opened for the selected buyer
• Close a channel from that list (no paste required)