Dropdown column selectors for all transform forms

[meghanagottapu]

Description

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.

Issue Link: Fixes #242

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

How Has This Been Tested?

Describe the tests you ran to verify your changes.

• Existing tests pass
• New tests added
• Manual testing

Screenshots (if applicable)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review
• I have added/updated documentation as needed
• My changes generate no new warnings
• Tests pass locally

[github-actions]

PR Review

Squash

Your PR has 5 commits. Please squash into a single commit.

How to fix

git fetch origin
git rebase -i origin/main   # mark all but first commit as "squash"
git push --force-with-lease

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