feat(desktop): add startup and onboarding flow

Add phase-based desktop boot progress, fresh-install sandbox testing, and first-run provider credential onboarding so packaged installs can start cleanly without manual settings detours.

apps/desktop/src/app/desktop-controller.tsx

@@ -3,6 +3,8 @@ import { useQueryClient } from '@tanstack/react-query'
 import { lazy, Suspense, useCallback, useEffect, useRef } from 'react'
 import { Navigate, Route, Routes, useLocation, useNavigate, useParams } from 'react-router-dom'
 
+import { DesktopBootOverlay } from '@/components/desktop-boot-overlay'
+import { DesktopOnboardingOverlay } from '@/components/desktop-onboarding-overlay'
 import { Pane, PaneMain } from '@/components/pane-shell'
 import { useSkinCommand } from '@/themes/use-skin-command'
 
@@ -395,6 +397,17 @@ export function DesktopController() {
 
   const overlays = (
     <>
+      <DesktopBootOverlay />
+      <DesktopOnboardingOverlay
+        enabled={gatewayState === 'open'}
+        onCompleted={() => {
+          void refreshHermesConfig()
+          void refreshCurrentModel()
+          void queryClient.invalidateQueries({ queryKey: ['model-options'] })
+        }}
+        onOpenSettings={openSettings}
+        requestGateway={requestGateway}
+      />
       <ModelPickerOverlay gateway={gatewayRef.current || undefined} onSelect={selectModel} />
 
       {settingsOpen && (