🛡️ Sentinel: Harden mod name and version validation against path traversal

[TheDragonSoft]

🚨 Severity: MEDIUM
💡 Vulnerability: Input validation for mod names and version strings (plausible_name and plausible_version) previously accepted strings containing ".." or leading/trailing dots.
🎯 Impact: Untrusted mod names or versions containing path traversal sequences ("..") could attempt path traversal when constructing API URLs, mirror download URLs, or local file names.
🔧 Fix: Updated plausible_name and plausible_version to explicitly reject strings containing ".." or starting/ending with dots.
✅ Verification: Added unit tests verifying rejection of ".." and leading/trailing dots and confirming valid mod names and semver strings are accepted. All 142 Rust tests pass and frontend build succeeds.

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PR created automatically by Jules for task 13291120663532169464 started by @TheDragonSoft

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