fix(ui): move the onboarding takeover chrome into the step

The settings shell painted the onboarding overlay (opaque stage, mask,
#root inert) the moment a step was registered and not locally completed,
while every step still had to load its private join before deciding
whether to show — rendering null could not suppress the shell-owned
chrome. Every reload on the hero therefore flashed a full-screen opaque
layer (white in the light palette) for one settings/credential RPC
round-trip after the session list turned ready.

The chrome now belongs to the step: a new zero-cordis OnboardingSurface
primitive (ui-primitives) renders the body-portaled overlay/mask/stage
verbatim from the former SettingsRoot stylesheet and holds #root inert
for exactly its own lifetime. WelcomeNotice and DeepSeekOnboardingDialog
wrap only their visible branch in it, so their existing null branches
paint and block nothing by construction. SettingsRoot keeps the
coordinator unchanged but renders the elected step bare, and the
settings.onboarding contract now names the surface wrap as the
registrant's obligation.

The onboarding e2e gains a held-join reload scenario pinning that a
configured world never mounts the takeover chrome or inerts the app.

apps/web/tests/onboarding-deepseek-config.e2e.ts

@@ -161,6 +161,49 @@ describe.skipIf(MODE === 'record')('web e2e: first-run DeepSeek credential setup
     expect(tripwire.pageErrors).toEqual([])
   }, 60_000)
 
+  it('never paints the takeover chrome on a configured reload, even with the settings join held open', async () => {
+    onTestFailed(() => saveFailureShot(page, 'web-e2e-onboarding-configured-reload'))
+    // Regression pin for the reload white flash: both steps are satisfied
+    // (welcome acknowledged, credential configured), yet each must LOAD its
+    // private join before it can decide not to show. The chrome lives inside
+    // the step (OnboardingSurface), so the deciding window paints and blocks
+    // nothing. Holding the join's settings.describe response widens that
+    // window from loopback-invisible to hundreds of milliseconds — without
+    // the hold, the assertions below would pass vacuously.
+    await page.addInitScript(() => {
+      const sightings: string[] = []
+      ;(window as unknown as { __takeoverSightings: string[] }).__takeoverSightings = sightings
+      setInterval(() => {
+        if (document.querySelector('[class*="onboardingStage"], [class*="onboardingMask"]') !== null) {
+          sightings.push('chrome')
+        }
+        if (document.getElementById('root')?.inert === true) sightings.push('inert')
+      }, 8)
+    })
+    let releaseDescribe = (): void => {}
+    const held = new Promise<void>((resolve) => { releaseDescribe = resolve })
+    let gated = false
+    await page.route('**/api/settings.describe', async (route) => {
+      if (gated) { await route.continue(); return }
+      gated = true
+      await held
+      await route.continue()
+    })
+    const warningsBefore = tripwire.warnings.length
+    await page.reload({ waitUntil: 'commit' })
+    await page.waitForSelector('[class*="frame"]', { timeout: 15_000 })
+    // The app is painted and interactive while the steps are still deciding.
+    await page.waitForTimeout(600)
+    releaseDescribe()
+    await page.waitForTimeout(400)
+    await page.unroute('**/api/settings.describe')
+    acknowledgeReloadConnectionLoss(tripwire, warningsBefore)
+    expect(await page.evaluate(() =>
+      (window as unknown as { __takeoverSightings: string[] }).__takeoverSightings)).toEqual([])
+    expect(await page.locator('[class*="onboardingStage"]').count()).toBe(0)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
   it('configures arbitrary DeepSeek models and prompts after the selected model is removed', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-onboarding-deepseek-models'))
     // Opened here rather than inherited: the credential test reloads the page