feat: improve model marketplace display

[laozuzhen]

Summary

• Reworks the available channels page into a model marketplace with group-first browsing, channel fallback, model icons, copy actions, and responsive layout.
• Extends available-channel responses with group image pricing tiers and displays original/effective prices using recharge and channel multipliers.
• Includes account-mapped/direct group models when channel model restrictions allow it, with tests for the expanded model listing behavior.

Test Plan

• pnpm build
• go test ./internal/service ./internal/handler ./internal/server -count=1
• go build ./cmd/server

[github-actions]

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