Refactor WebKit URL validation to use libcurl

[BlythT]

To prevent #803 happening again, I refactored the URL validation:

• Extracted validation logic into a class target_url
• Replaced the existing regex and string matching with libcurl (already imported so no added dependencies) for more robust URL parsing
• Added exhaustive testing to detect automatically if this breaks in future

I wanted to add ctest --test-dir build --output-on-failure -C Release to the github actions ci.yml but I encountered unrelated test failures which would block build if merged https://github.com/BlythT/Millennium/actions/runs/27465672305 so I've reverted that