fix: keep video remix controls editable

[atomantic]

Summary

• Moves a remix from a fixed-profile source model to a compatible editable text model, preserving the restored negative prompt, Steps, and CFG Scale.
• Keeps source-model remixes with substitute text encoders or LoRAs intact so model-specific conditioning is never dropped.
• Explains the automatic model change directly below the selector.

Test plan

• npm test --prefix client -- --run src/hooks/useVideoGenForm.test.jsx src/components/videoGen/AdvancedParamsPanel.test.jsx src/pages/VideoGen.terms.test.jsx src/pages/VideoGen.textEncoderAutoDownload.test.jsx
• npm run lint --prefix client
• npm run build --prefix client