Make URL input accept "example.com" without requiring "https://"

[ieliofficial]

Currently, the URL input at frontend/src/landingPage.jsx uses new URL(url) which throws an error for URLs like "google.com" - it only accepts full URLs with protocol (e.g., "https://google.com/").

Requirements:

1. Modify the handleScan function to prepend "https://" to the input if no protocol is provided
2. After normalization, validate that the URL is valid
3. Update the error message to be more helpful (e.g., "Please enter a valid website address")
4. Ensure tests in frontend/src/tests/landingPage.test.jsx pass with new behavior

Test cases to add:

• "google.com" should work
• "example.org" should work
• "localhost:3000" should work
• Invalid inputs like "not-a-url" should still show error