Fixes edge case with absolute path

[Vein05]

Issue: url.ParseRequestURI() incorrectly handles absolute file paths as valid URLs

Problem
When using url.ParseRequestURI() to validate URLs for image processing, the function incorrectly returns success (nil error) for absolute file paths (e.g., /usr/local/images/photo.jpg) because it considers them valid URI paths according to RFC 3986. This caused the code to attempt downloading local files as if they were web URLs.

Impact

• Local file paths were being mistakenly treated as web URLs
• Unnecessary HTTP requests were being made for local files
• Failed image processing when using absolute paths

Solution
Replace the url.ParseRequestURI() check with an explicit HTTP(S) prefix check:

Technical Details

• The URI specification (RFC 3986) considers absolute paths starting with "/" as valid URI paths
• url.ParseRequestURI() follows this specification, making it unsuitable for distinguishing between web URLs and file paths
• Using strings.HasPrefix() provides a more explicit and accurate way to identify web URLs

Testing

• Test with absolute file paths (e.g., /usr/local/images/photo.jpg)
• Test with relative file paths (e.g., ./images/photo.jpg)
• Test with HTTP URLs (e.g., http://example.com/photo.jpg)
• Test with HTTPS URLs (e.g., https://example.com/photo.jpg)

Resolves: #54