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>

frontend/src/components/layout/Sidebar.jsx

@@ -68,6 +68,7 @@ const Icons = {
   commandCenter:    () => <S><rect x="2" y="2" width="12" height="12" rx="1.5"/><path d="M5 6l2 2-2 2M9 10h2"/></S>,
   blackBox:         () => <SvgIcon Component={IcoBlackbox} />,
   deviceSettings:   () => <SvgIcon Component={IcoMelodySettings} />,
+  onboard:          () => <S><path d="M5 8l2 2 4-4"/><rect x="2" y="2" width="12" height="12" rx="2"/></S>,
   appUsers:         () => <S><circle cx="6" cy="5" r="2.5"/><path d="M2 13c0-2.2 1.8-4 4-4"/><circle cx="12" cy="7" r="2"/><path d="M9.5 13c0-1.65 1.12-3 2.5-3s2.5 1.35 2.5 3"/></S>,
   melodies:         () => <SvgIcon Component={IcoMelodies} />,
   library:          () => <SvgIcon Component={IcoMelodiesEditor} />,
@@ -132,6 +133,7 @@ const navSections = [
       { to: '/mqtt/commands',     label: 'Command Center',  icon: 'commandCenter' },
       { to: '/equipment/notes',   label: 'BlackBox',        icon: 'blackBox' },
       { to: '/devices/settings',  label: 'Device Settings', icon: 'deviceSettings',  placeholder: true },
+      { to: '/devices/onboard',   label: 'Onboard',         icon: 'onboard' },
     ],
   },
   { to: '/users', label: 'App Users', icon: 'appUsers', permission: 'app_users' },