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>

appnav/src/main/kotlin/io/element/android/appnav/LoggedInFlowNode.kt

@@ -113,6 +113,7 @@ import java.time.Duration
 import java.time.Instant
 import java.util.Optional
 import java.util.UUID
+import kotlin.time.Duration.Companion.milliseconds
 import kotlin.time.Duration.Companion.minutes
 import kotlin.time.Duration.Companion.seconds
 import kotlin.time.toKotlinDuration
@@ -324,7 +325,9 @@ class LoggedInFlowNode(
 
     override fun resolve(navTarget: NavTarget, buildContext: BuildContext): Node {
         return when (navTarget) {
-            NavTarget.Placeholder -> emptyNode(buildContext)
+            // Rendered while the FtueState is Unknown. Render a loading state rather than nothing at all,
+            // since there is no guarantee that this state will be left.
+            NavTarget.Placeholder -> emptyNode(buildContext, delayBeforeShowingContent = 500.milliseconds)
             NavTarget.LoggedInPermanent -> {
                 val callback = object : LoggedInNode.Callback {
                     override fun navigateToNotificationTroubleshoot() {