feat(devices): add device onboarding wizard
New guided flow (Bell Cloud > Devices > Onboard) for claiming a manufactured/flashed device into a customer's fleet in one pass: looks up the device by serial, finds or creates the customer (CreateCustomerModal for a quick inline create), finds or creates the app user account (reuses CreateUserModal from the user-creation work), and assigns the device — replacing what used to require jumping between the inventory, CRM, and user-management pages separately. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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