feat: share channels with interoperable QR URLs

[RCGV1]

Summary

• generate canonical ChannelSet QR URLs instead of PSK-only URLs
• make Replace the default and include LoRa only for Replace
• make Add use ?add=true and omit LoRa so receiver settings are preserved
• retain reboot feedback for actual region/modem changes

Part of meshtastic/design#128. Fixes #350.

Validation

• focused channel_share_tests --no-skip: 2 tests, 16 assertions
• independent review verified the protobuf wire format against the Apple/Android/Web contract

Remaining validation

• full CMake/CTest was blocked locally by LovyanGFX bootstrap and missing grpc-tools
• no hardware TFT modal/event-flow validation yet

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