🛡️ Sentinel: Fix path traversal vulnerability in pack saving

[TheDragonSoft]

🚨 Severity: HIGH
💡 Vulnerability: Missing pack ID validation in save_pack allowed arbitrary file paths / path traversal when saving pack manifests.
🎯 Impact: An invalid pack ID containing directory traversal sequences (e.g. ../) could allow writing JSON files outside the intended profiles directory.
🔧 Fix: Added valid_id(&pack.id) validation in save_pack in src-tauri/src/core/services/packs.rs, matching load_pack and delete_pack.
✅ Verification: Added unit test save_pack_rejects_invalid_pack_id_and_path_traversal verifying path traversal, slashes, and empty IDs are rejected.

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PR created automatically by Jules for task 16792835397477577662 started by @TheDragonSoft

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