Fix: Resolve multiple TODOs across the codebase

[blopa]

This commit addresses a large number of TODO items found throughout the project.

Key changes include:

• Refactored utility functions in src/utils/utils.js (convertToToggleableObject, convertToRegularObject) to ensure immutability by operating on cloned objects.
• Fixed hardcoded labels in ResumeDrawerItems components, now using varNameToString for dynamic label generation.
• Addressed a NODE_ENV workaround in gatsby-node.js by commenting it out and improving related comments.
• Improved code documentation for URL parsing regex in src/utils/spreadsheet-parser.js.
• Completed TemplateSelector and LanguageSelector components by adding missing PropTypes.
• Clarified parameter/URI handling and prop passing in ResumeViewer.jsx with comments.
• Implemented a user alert in DropZone.jsx when the file upload limit is exceeded.
• Added basic Formik onSubmit (logging) and validate (name field required) logic in BuildPage.jsx.
• Removed an unused baseTheme object from src/themes/theme.js.
• Updated README.MD with documentation on how to create new resume templates.
• Cleaned up and removed obsolete TODO sections from older README_V1.MD, README_V2.MD, and README_V3.MD.
• Added client-side validation for Google Spreadsheet URLs in UploadPage.jsx, displaying a Snackbar error for invalid formats.
• Implemented user-friendly error messages in ResumeViewer.jsx for issues during data fetching or processing.
• Created src/utils/utils.test.js with basic unit tests for key utility functions, covering immutability and core logic.

Note: New internationalization keys added for error messages in DropZone.jsx, UploadPage.jsx, and ResumeViewer.jsx could not be added to src/intl/fr.json due to a persistent error I encountered. This file will require manual updating for the new keys.

[blopa]

Disclaimer: This PR was implemented by https://jules.google.com/