feat(flows): support variable templates in Google Sheets column mapping

[viethung0o0]

Summary

Google Sheets Send Data and Update Row steps can now write a variable/text template to each column, instead of only mapping a single contact custom field. Existing flows keep working unchanged and write exactly the same values.

Details

• Column values are authored with the variable text editor (custom fields, system fields, free text) instead of a custom-field dropdown.
• Steps are versioned: existing steps run on the original path; opening a node upgrades them to the new format, preserving the values written to the sheet (including date/datetime).
• A tag-only backfill script marks existing steps; runtime correctness does not depend on it.
• Also groups multiple spreadsheet step dialogs so only one is open at a time.

Test plan

• flow-config, variables, and worker unit tests (versioning, value resolution, nested steps, no extra queries)
• Type checks: builder, worker, flow-config, database
• Open an existing Google Sheets flow, confirm columns still receive the same values
• Create a new step, map columns with variables, confirm values are written