Show a loading indicator on the placeholders which can stay on screen forever

emptyNode() renders nothing at all, which is fine for a placeholder that is left
within a few frames, but makes the application look frozen when the wait can
last, or never end.

Add a delayBeforeShowingContent parameter to emptyNode(): when it is finite, a
progress indicator and a "Loading…" label fade in after that delay, reusing the
existing DelayedVisibility helper. It defaults to Duration.INFINITE, so the
splash screen keeps rendering an empty canvas.

Use a one second delay on the two placeholders which are not guaranteed to be
left: the FTUE one, rendered while waiting for the session verified status, and
the LoggedInFlowNode one, rendered while the FTUE state is unknown.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

features/ftue/impl/src/main/kotlin/io/element/android/features/ftue/impl/FtueFlowNode.kt

@@ -37,6 +37,7 @@ import kotlinx.coroutines.flow.filterIsInstance
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
 import kotlinx.parcelize.Parcelize
+import kotlin.time.Duration.Companion.milliseconds
 
 @ContributesNode(SessionScope::class)
 @AssistedInject
@@ -84,7 +85,10 @@ class FtueFlowNode(
     override fun resolve(navTarget: NavTarget, buildContext: BuildContext): Node {
         return when (navTarget) {
             NavTarget.Placeholder -> {
-                emptyNode(buildContext)
+                // Rendered while waiting for the initial state (see FtueStep.WaitingForInitialState).
+                // This can last for a while, or forever if the sync service never manages to run, so
+                // render a loading state rather than nothing at all.
+                emptyNode(buildContext, delayBeforeShowingContent = 500.milliseconds)
             }
             is NavTarget.SessionVerification -> {
                 val callback = object : FtueSessionVerificationFlowNode.Callback {