fix: add URL format validation for learning resources

[nancysangani]

Fixes #781

Learning resources in projects.json are stored as "Label: URL" strings with no validation. Malformed URLs were passed straight to the template as raw anchor tags, resulting in broken links with no indication to the user.

Changes:

• utils/url_validator.py — new module: parse_resource, is_valid_url, validate_resource, validate_resources; format-only validation (no outbound HTTP), regex rejects bare domains, wrong schemes, empty strings
• utils/data_loader.py — validate_projects() now calls parse_resource + is_valid_url on every resource entry and logs a warning for malformed URLs at startup; raises are intentionally avoided so a single bad URL in the JSON doesn't crash the app
• routes/main_routes.py — new GET /api/project/<id>/resources route returning [{label, url, valid}] structured objects; frontend can use the valid flag to render broken links with a warning instead of a dead anchor
• tests/test_resource_url_validation.py — 45 tests covering validator edge cases, all current projects.json URLs, route response shape, 404 handling, security headers

[vercel]

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[nancysangani]

Hi @komalharshita, please review the PR when you get a chance. Thanks!