🛡️ Sentinel: [HIGH] Fix path traversal in employee portrait loading

[mleem97]

🚨 Severity: HIGH
💡 Vulnerability: Path traversal vulnerability in SetPortrait where employeeId was used directly in Path.Combine without validation, relying entirely on upstream validation during registration. This could be bypassed via direct calls or deserialization.
🎯 Impact: An attacker could potentially access or read arbitrary files outside the intended ModAssets directory by supplying an employeeId containing directory traversal sequences (e.g., ../).
🔧 Fix: Added an independent validation check for employeeId directly within SetPortrait before attempting file system access. The vulnerable operation is wrapped in a conditional block to ensure fallback UI logic remains reachable even if validation fails.
✅ Verification: Ran dotnet test tests/gregCore.Tests.csproj to confirm the code compiles successfully and no new test failures are introduced.

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PR created automatically by Jules for task 5917803949505319775 started by @mleem97

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[codacy-production]

Up to standards ✅

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