Support Intel Mac and dual USB ID (DJI/Quectel)

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Summary

• Add Intel Mac (x86_64) support via unified build script with auto architecture detection
• Support both USB IDs simultaneously: 2ca3:4006 (DJI original) and 2c7c:0125 (Quectel EC25 flashed)
• Remove arm64-only restriction from packaging/install
• Update README with Intel Mac build instructions
• Add debug logging for USB AT discovery troubleshooting

Changes

• usbat_darwin.go: Replace hardcoded USB ID constants with supportedUSBIDs table
• main.go: Update discoverDJIUSBDevice() to recognize both vendor IDs
• scripts/build-macos.sh: Unified build script with auto architecture detection
• packaging/install: Accept both arm64 and x86_64
• README.md & packaging/README.md: Document Intel Mac support

Test

• Built and tested on Intel Mac (x86_64)
• USB AT communication established
• SMS receiving working
• eSIM manager available
• Apple Silicon verification (help wanted)