fix(desktop): mount onboarding from frame 1 to kill the FOUT

Default onboarding.configured to null (unknown until the runtime check resolves) and have the onboarding overlay render whenever it's not yet confirmed true. The boot overlay now yields to it, so the very first paint is the Welcome card with a "While we get you set up..." progress strip instead of a flash of the chat shell between boot dismiss and onboarding mount.

The picker swaps in cleanly once the gateway opens and the runtime check confirms the user is not configured. Already-configured users see the same prep card briefly while their existing runtime warms up, then the overlay dismisses without touching the chat shell.

apps/desktop/src/store/onboarding.ts

@@ -27,7 +27,9 @@ export type OnboardingFlow =
   | { message: string; provider?: OAuthProvider; start?: OAuthStartResponse; status: 'error' }
 
 export interface DesktopOnboardingState {
-  configured: boolean
+  /** null until the first runtime check resolves; lets the overlay render
+   *  during boot without flickering for already-configured users. */
+  configured: boolean | null
   flow: OnboardingFlow
   mode: OnboardingMode
   providers: null | OAuthProvider[]
@@ -41,7 +43,7 @@ export interface OnboardingContext {
 }
 
 const INITIAL: DesktopOnboardingState = {
-  configured: true,
+  configured: null,
   flow: { status: 'idle' },
   mode: 'oauth',
   providers: null,