CrossPad PC v0.3.0

What's Changed

• fix: Checkout to subfolder to preserve release archives
• fix: Move checkout before archive creation in release workflow
• fix: Use _Exit() instead of _exit() for Linux compatibility
• feat: Virtual USB/UART with COM port backend, Serial Monitor app, auto-connect
• fix: Graceful window close via _exit() to avoid FreeRTOS thread abort
• fix: Add allowed_mentions for Discord role pings

Full Changelog: v0.3.0-CR...v0.3.0