refactor(spectra): submit payload for multi-save with spectra_list

[Nicolass67]

Summary

This PR updates submit payload handling to support multi-save in a consistent way.

• Refactored the spectra save flow to make saveOp smaller and easier to read.
• Split save logic into focused helpers (getSavePayloads, buildSerializedPayload, saveTarget, combineBatchIfNeeded).
• Moved combine API call from ViewSpectra to SpectraActions.CombineSpectra for cleaner separation.
• Kept combine generation only for accepted multi-layouts and only when batch conditions are met.
• Use spectra_list as the submit payload format.
• Apply the same submit flow for single and multi spectra.
• Ensure each spectrum item contains its own required save data.