feat: add URL normalization and validation (reject non-http/https)

[Mojirade18]

🔧 Fix: Validate Extracted URLs

Description
This PR adds validation for extracted URLs before saving them. The main goals are:

• Prevent broken links from being stored.
• Avoid security risks from javascript: and other invalid URLs.
• Improve reliability of collected leads.

Changes Made

• Added a URL validation function.
• Filtered out invalid URLs (null, undefined, javascript:, malformed links).
• Updated the extraction logic to ensure only valid URLs are included.

Testing

• Extracted leads from multiple pages with a mix of valid and invalid links.
• Confirmed that broken or malicious URLs are excluded.
• Verified that the extension still collects correct leads as expected.

Related Issue
Closes #5 (Unvalidated URLs issue).

Checklist

• Code follows project guidelines.
• Tests performed successfully.
• Documentation updated if necessary.