fix: restore shared market web sessions

[bobleer]

Summary

• deploy the completed shared MiniApp/Skin account contract as one compatible revision
• use the canonical camelCase OAuth return target from both market frontends
• keep the legacy snake_case query parameter accepted server-side
• document the cross-market deployment compatibility requirement

Verification

• cargo test -p bitfun-miniapp-market-service
• cargo test -p bitfun-skin-market-service
• cargo check -p bitfun-miniapp-market-server
• cargo check --workspace
• MiniApp market type-check, 10 tests, production build
• Skin market type-check, 14 tests, production build
• production MiniApp and Skin containers healthy at 30c3a76
• browser verified @bobleer on /skin/submissions and /skin/admin

AI-assisted: fully tested and deployed to the existing production market services.