ui: fix crash opening About SimpleX Chat (#6902)

* ui: fix crash and logo cutoff on About SimpleX Chat

OnboardingShrinkingLayout calls .first() on each slot's measurables,
crashing when the button slot is empty. About passes onboardingStage =
null, in which case the button slot emits no children. Have the About
branch emit a 0x0 Spacer so the slot always has one measurable.

Also add a top inset for the SimpleX logo when reached from About in
non-oneHandUI mode — without it the top app bar overlaps the logo.

* style

Co-authored-by: Evgeny <evgeny@poberezkin.com>

* Apply suggestion from @epoberezkin

Co-authored-by: Evgeny <evgeny@poberezkin.com>

---------

Co-authored-by: Evgeny <evgeny@poberezkin.com>

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/onboarding/SimpleXInfo.kt

@@ -105,7 +105,9 @@ fun SimpleXInfoLayout(
   user: User?,
   onboardingStage: SharedPreference<OnboardingStage>?
 ) {
-  Column(Modifier.fillMaxSize().systemBarsPadding().padding(horizontal = DEFAULT_ONBOARDING_HORIZONTAL_PADDING), horizontalAlignment = Alignment.CenterHorizontally) {
+  val topBar = onboardingStage == null && !appPrefs.oneHandUI.state.value
+  val modifier = Modifier.fillMaxSize().systemBarsPadding().padding(horizontal = DEFAULT_ONBOARDING_HORIZONTAL_PADDING)
+  Column(if (topBar) modifier.padding(top = AppBarHeight * fontSizeSqrtMultiplier) else modifier, horizontalAlignment = Alignment.CenterHorizontally) {
     Box(Modifier.padding(top = DEFAULT_PADDING * 2).widthIn(max = if (appPlatform.isAndroid) 185.dp else 160.dp), contentAlignment = Alignment.Center) {
       SimpleXLogo()
     }
@@ -156,6 +158,8 @@ fun SimpleXInfoLayout(
             ModalManager.fullscreen.showModal { HowItWorks(user, onboardingStage) }
           })
         }
+      } else {
+        Spacer(Modifier)
       }
     }
   )