Add SIGINT handler and block app exit during firmware updates

[ABuljko]

• Custom SIGINT handler Ctrl+C now gracefully closes the app instead of being ignored (fixes Handle signals properly #132)
• Signal wakeup timer periodically hands control back to the Python interpreter so signals get delivered while the Qt event loop runs
• Update guard blocks exit (Ctrl+C and window close) while a firmware update is running, preventing device damage
• Shows a warning message when an exit is blocked during an update