fix(desktop): flip e2e assertion + exempt boot-failure from error guard

- large-session-reload: assert exactly 1 burst (was >= 2) — the fix
  eliminates the duplicate re-render, so 1 is the expected count
- boot-failure: add allowErrorBanners() beforeEach — these tests
  deliberately trigger boot errors, so error toasts are expected
- test.ts: export allowErrorBanners() opt-out + reset flag in afterEach

apps/desktop/e2e/boot-failure.spec.ts

@@ -9,7 +9,7 @@
  * Prerequisite: `npm run build` must have been run so dist/ exists.
  */
 
-import { test } from './test'
+import { allowErrorBanners, test } from './test'
 
 import {
   type DeadBackendFixture,
@@ -26,6 +26,12 @@ test.afterAll(async () => {
 })
 
 test.describe('boot failure with dead backend', () => {
+  test.beforeEach(() => {
+    // These tests deliberately trigger boot errors — error banners
+    // (notifyError → [role="alert"]) are expected, not failures.
+    allowErrorBanners()
+  })
+
   test('app shows error state', async () => {
     // Inject a fake boot error so the backend resolution "fails" with a
     // controlled error message. This is the only reliable way to trigger

apps/desktop/e2e/large-session-reload.spec.ts

@@ -337,11 +337,15 @@ test.describe('loading a large previous session', () => {
     expect(results, 'render count data should have been collected').not.toBeNull()
 
     if (results) {
+      // After the fix, the transcript should paint exactly once (1 burst).
+      // Before the fix, it painted 2-3 times (the "re-renders a few times
+      // as it loads" bug). This assertion proves the fix works — if it
+      // ever regresses to >= 2, the test will fail.
       expect(
         results.bursts,
-        `expected >= 2 mutation bursts (multiple re-renders during load), ` +
+        `expected exactly 1 mutation burst (single paint after fix), ` +
           `got ${results.bursts}: ${JSON.stringify(results.timeline)}`,
-      ).toBeGreaterThanOrEqual(2)
+      ).toBe(1)
     }
   })
 })

apps/desktop/e2e/test.ts

@@ -18,6 +18,18 @@ import { test as base, expect, type Page, type ElectronApplication, _electron }
 // Track error messages per test so afterEach can assert + report.
 const seenErrors: string[] = []
 let activePage: Page | null = null
+// When true, the afterEach guard skips the error-banner check.
+// Set by tests that deliberately trigger error states (e.g. boot-failure).
+let errorBannersAllowed = false
+
+/**
+ * Opt out of the error-banner guard for the current test. Call in
+ * test.beforeEach or at the top of a test body when error banners are
+ * expected (e.g. boot-failure tests that deliberately trigger errors).
+ */
+export function allowErrorBanners(): void {
+  errorBannersAllowed = true
+}
 
 /**
  * Install the error-banner guard on a page. Watches for `[role="alert"]`
@@ -125,6 +137,16 @@ export const test = base.extend({
 // default `page` fixture — Electron tests create their own page via
 // app.firstWindow(), so the default `page` fixture is undefined.
 base.afterEach(async ({}, testInfo) => {
+  const wasAllowed = errorBannersAllowed
+  // Reset for the next test.
+  errorBannersAllowed = false
+
+  if (wasAllowed) {
+    // Test opted out — clear any collected errors without asserting.
+    seenErrors.length = 0
+    return
+  }
+
   const errors = await collectErrorBanners(activePage)
 
   if (errors.length > 0) {