Add Doubao provider support

[Manho]

Summary

• add Doubao as a first-class provider across manifest, provider metadata, options, docs, and locales
• wire Doubao-specific text, send, image upload, and enter-key handling based on the live web app behavior
• add provider migration logic and extend layout support with a manual 1x8 option while keeping 1x7 -> 4x2 auto-upgrade

Testing

• npm test -- --run tests/provider-defaults.test.js tests/service-worker.test.js tests/doubao-content-script.test.js tests/layout-auto-adjust.test.js tests/manifest-doubao-content-script.test.js tests/providers.test.js tests/settings.test.js
• npx playwright test tests/e2e/layout-auto-adjust.e2e.test.js