Fix Issue #2042: prevent invalid input alert on backspace in Armstrong Checker

[Euphoric-Coder]

Issue (Number)

Closes #2042

---

Description

This PR fixes the invalid input alert that appeared when users pressed Backspace on an empty or single-digit input in the Armstrong Number Checker project.
The previous implementation triggered an intrusive alert() and page reload even during normal text editing, leading to a poor user experience.

Changes Made

• Removed inline onkeyup="textfxn()" from armstrong.html
• Replaced alert-based validation with inline error handling in armstrong.js
• Disabled validation during normal typing and deletion
• Displayed non-blocking feedback message (⚠️ Please enter a valid positive number.)
• Improved readability and user experience consistency

---

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

---

Checklist

• I have made this from my own
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• The title of my pull request is a short description of the requested changes

---

ATTACH SCREEN-SHOTS / DEPLOYMENT LINK

Before:
An alert popup saying “Invalid input!” appeared when pressing Backspace on empty/single-digit input.

After:
No alert interruption. Inline message appears under the input box, maintaining a smooth typing flow.

Deployment Link:
https://frontendprojects.netlify.app/projects/armstrongchecker/armstrong