feat: add dual frame webcam layout preset

[shreyas-makes]

Summary

• add a new Dual Frame webcam layout preset with a 2:1 screen-to-camera split for landscape compositions
• preserve aspect ratios by rendering the screen share to fill the left pane and center-cropping the webcam to fit the right pane in preview and export
• update editor preset selection, persistence, localized labels, and layout tests for the new preset

Testing

• npx vitest --run src/lib/compositeLayout.test.ts src/components/video-editor/projectPersistence.test.ts
• npx biome check src/lib/compositeLayout.ts src/components/video-editor/videoPlayback/layoutUtils.ts src/lib/exporter/frameRenderer.ts src/lib/compositeLayout.test.ts

[shreyas-makes]

Closing this duplicate PR in favor of the upstream PR at siddharthvaddem#347.