Show Preprocessor for all pipelines and fix VAE disabled state

[yondonfu]

Summary

• Show Preprocessor selector for all pipelines, not just VACE pipelines (backend already supports preprocessor → non-VACE pipeline routing via latent initialization)
• Fix VAE control to be disabled while pipeline is loading, matching Preprocessor and Postprocessor behavior

The rationale for always showing the Preprocessor selector is that even though a typical preprocessor right now would be used to create control video for a VACE enabled pipeline there is not a strict requirement that preprocessors only be used with VACE - for example, you could create a Preprocessor that automatically adjusts the brightness of incoming video which is compatible with non-VACE pipelines too. Additionally, an asymmetry where Postprocessor selector is always shown but Preprocessor is not always shown feels strange in the UI layout.

Test plan

• Run npm run build in frontend directory - build succeeds
• Start app with uv run daydream-scope
• Verify Preprocessor selector appears for non-VACE pipelines
• Verify VAE, Preprocessor, and Postprocessor controls are all disabled during pipeline loading

🤖 Generated with Claude Code