feat: add VirusTotal process scan via SHA-256 hash

[IslamAbdelslam]

Adds on-demand security scanning for any running process by:

• Hashing the process executable (SHA-256 via /proc//exe)
• Querying the VirusTotal v3 API with the computed hash
• Displaying a colour-coded verdict in a dedicated modal

Backend (Rust)

• New virustotal module with hash_executable() and check_virustotal() functions
• Two new Tauri commands: hash_process and check_virustotal_hash
• Graceful error handling: permission denied, rate limits (429), invalid API key (401), file not in database (404)
• Reads executable in 64 KiB chunks to avoid loading large binaries fully into memory

Frontend (Svelte)

• New VirusTotalModal component with three phases: idle → loading (hashing / querying) → result
• Colour-coded verdict badge using existing CSS palette: green (clean), red (malicious), yellow (suspicious), grey (unknown)
• SHA-256 hash display + per-engine stats grid
• API key persisted to localStorage; never sent to the backend at rest
• "View on VirusTotal" opens the system browser via the shell plugin
• New teal shield button added to per-row action buttons

Tests

• 9 new unit tests covering: SHA-256 format, determinism, invalid PIDs, empty/whitespace API key rejection, VTReport JSON serialisation, and all four verdict values
• All 13 tests (including pre-existing ones) pass