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>

libraries/ui-common/build.gradle.kts

@@ -17,4 +17,5 @@ android {
 dependencies {
     implementation(libs.appyx.core)
     implementation(projects.libraries.designsystem)
+    implementation(projects.libraries.uiStrings)
 }