fix: firewall shutdown on ctrl+c signal

[Satvik-Singh192]

Fix Firewall Shutdown on Ctrl+C

Description

Fixed firewall process termination issue where repeated Ctrl+C inputs would trigger shutdown messages but the program would hang indefinitely.
Implemented robust signal handling with timeout mechanisms and proper thread management to ensure clean shutdown.

---

Changes Made

🔧 Core Fixes

• Enhanced signal handler in main.py with timeout protection and state tracking
• Improved thread management in core.py with coordinated shutdown using _stop_event
• Better packet capture control using AsyncSniffer with immediate termination
• Graceful degradation with force exit after timeout for stuck threads

🗂️ Files Modified

• main.py – Added enhanced signal handler with global state tracking
• core.py – Implemented proper thread termination and sniffer control

✨ New Features

• Timeout protection: 5-second graceful shutdown window before force exit
• State tracking: Prevents multiple signal handlers from interfering
• Interruptible threads: Background threads check shutdown flag frequently
• Force exit capability: Handles stubborn threads that don't terminate

---

Semver Changes

• Patch (bug fix, no new features)
• Minor (new features, no breaking changes)
• Major (breaking changes)

---

Issues

Closes #1 – Firewall process does not terminate on Ctrl+C

---

• I have read the Contributing Guidelines.

[Satvik-Singh192]

@utkarshkgithub please review my PR sir