Replace UF2 file copy to mass storage volume with direct picotool upload.
This avoids macOS "disk not ejected properly" warnings caused by the RP2040
resetting immediately after receiving the firmware.
- Use picotool (already installed by PlatformIO) for BOOTSEL detection
and firmware upload via USB
- Upload ELF directly with `picotool load -v -x` for real-time progress
- Remove platform-specific mass storage volume detection (macOS/Linux/Windows)
- Show helpful udev rules message on Linux permission errors