This implements the WebUSB-based web installer using fastboot.js to
act as a fastboot client. A bare minimum UI with a plain-text
status/progress caption for each step is included, as well as a plain
button to trigger it and basic error handling.
WebADB has been removed now that we are only using fastboot.js.
Initial features:
- Unlocking and locking the bootloader
- Downloading the latest GrapheneOS release available for the device
- Flashing the factory images zip
- Reusing USB connections
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