test(onboarding): widen onboarding E2E waitForText timeout to 15s

OnboardingFlowTest passed on the API-37 job but timed out (ComposeTimeoutException
after 5000ms) across the animation-disabled API 29-36 matrix, at the single
async-gated transition: click -> viewModelScope coroutine -> addImapAccount ->
DONE -> LaunchedEffect -> navigate -> AddAnother render. The flow is correct
(green on API 37; ManualSetupScreenTest proves the add-callback path); the 5s cap
was just too tight for that compound step on slower matrix emulators. waitUntil
returns as soon as the text appears, so the happy path is unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/androidTest/kotlin/org/libremail/ui/onboarding/OnboardingFlowTest.kt

@@ -52,7 +52,11 @@ class OnboardingFlowTest {
 
     private fun string(resId: Int) = composeTestRule.activity.getString(resId)
 
-    private fun waitForText(text: String) = composeTestRule.waitUntil(5_000) {
+    // 15s (not the 5s used elsewhere): the add-another step is reached only after an async chain
+    // (button → viewModelScope coroutine → addImapAccount → DONE → LaunchedEffect → navigate), which
+    // on the slower, animation-disabled E2E matrix emulators can exceed a 5s budget for that one
+    // transition. waitUntil returns as soon as the text appears, so this only raises the cap.
+    private fun waitForText(text: String) = composeTestRule.waitUntil(15_000) {
         composeTestRule.onAllNodesWithText(text).fetchSemanticsNodes().isNotEmpty()
     }