Add client-side TF-IDF full-text search engine (closes #806)

[dikshajangra12918-oss]

Description

Implements a client-side full-text search engine with TF-IDF based relevance ranking, built entirely as a single self-contained HTML file — no backend, no database, no external libraries.

Features

• Paste multiple documents/paragraphs to build an in-browser inverted index
• TF-IDF scoring (normalized term frequency × smoothed inverse document frequency) computed entirely client-side
• Search bar returns ranked results with relevance scores displayed
• Matched terms highlighted within result snippets for context
• Add/remove documents dynamically, with a "Load Sample Docs" option for quick testing
• Fully offline — works directly from file://, zero dependencies

Testing

Tested locally in browser — indexed sample documents, ran multiple queries, verified ranking order, score display, and term highlighting all work as expected.

Closes #806

[praveenscience]

Kindly follow the contribution guidelines. Also honour the PR task list please.

[praveenscience]

Create a new PR with the following insturctions:

Procedure

1. Make a great design & think of a style, say retro.
2. Go to main branch of the repo.
3. Pull the upstream branch: git pull upstream main.
4. Copy the contents of the HTML.
5. Inside the tools, create the file github-token-validator-retro.html.
6. Paste the contents of the HTML.
7. Take a screenshot of the UI.
8. Save it as github-token-validator-retro.png.
9. Edit the tools.json with the newly created github-token-validator-retro.
10. Fill in all the fields.
11. Update the ReadMe with the newly created file.
12. Run two commands: node build and node sort-norm tools.json.
13. Commit the files.
14. Raise the Pull Request.
15. Get it merged!