Fix Doubao release review findings

[Manho]

Summary

• Preserve reordered legacy provider defaults when migrating users to include Doubao.
• Require a rendered Doubao image preview before treating image injection as successful.
• Prevent auto-submit when Doubao image injection fails and avoid repeated failed uploads.

Verification

• git diff --check origin/main...HEAD
• npx vitest --run tests/provider-defaults.test.js tests/service-worker.test.js tests/doubao-content-script.test.js