[Bug] Dropdown Column Selectors for All Transform Forms

[meghanagottapu]

Describe the bug
Replaces free-text column name inputs with <select> dropdowns in all 6 transform forms. Column names are sourced from ProjectContext (already available globally) So no extra API calls are needed.

To Reproduce
Steps to reproduce the behavior:

1. Upload a CSV
2. Navigate to workspace
3. open filter form

Expected behavior
What you expected to happen.
Previously, users had to know column names exactly and type them manually. Typos produced 400 confusing errors. This change is explicitly listed as an Expected.

'All transform forms use dropdown column selectors instead of free-text input'

Screenshots
If applicable, add screenshots.

Environment:

• OS: macOS
• Browser: Safari
• Node.js version: v24.12.0
• Python version: 3.14.2