feat(flutter): DelayedLoading wrapper for skeleton-with-delay

Renders nothing for the first [delay] (default 200ms) of sustained
loading, then renders the [whileDelayed] child until loading completes.
Mirrors the web useDelayed hook so a fast network response doesn't
flash a skeleton.

Used in the next commit's home screen rewrite; otherwise unused this
slice.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/shared/delayed_loading.dart

@@ -0,0 +1,66 @@
+import 'dart:async';
+import 'package:flutter/widgets.dart';
+
+/// Renders [whileDelayed] only after [isLoading] has been true
+/// continuously for [delay], then renders [whenReady] once loading
+/// completes. Mirrors the web `useDelayed` hook: a brief loading flash
+/// doesn't trigger a skeleton; sustained loading does.
+///
+/// Once [isLoading] flips back to false, the timer resets.
+class DelayedLoading extends StatefulWidget {
+  const DelayedLoading({
+    super.key,
+    required this.isLoading,
+    required this.whileDelayed,
+    required this.whenReady,
+    this.delay = const Duration(milliseconds: 200),
+  });
+
+  final bool isLoading;
+  final Widget whileDelayed;
+  final Widget whenReady;
+  final Duration delay;
+
+  @override
+  State<DelayedLoading> createState() => _DelayedLoadingState();
+}
+
+class _DelayedLoadingState extends State<DelayedLoading> {
+  Timer? _timer;
+  bool _delayPassed = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _maybeStart();
+  }
+
+  @override
+  void didUpdateWidget(DelayedLoading old) {
+    super.didUpdateWidget(old);
+    if (widget.isLoading != old.isLoading) {
+      _timer?.cancel();
+      _delayPassed = false;
+      _maybeStart();
+    }
+  }
+
+  void _maybeStart() {
+    if (!widget.isLoading) return;
+    _timer = Timer(widget.delay, () {
+      if (mounted) setState(() => _delayPassed = true);
+    });
+  }
+
+  @override
+  void dispose() {
+    _timer?.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (!widget.isLoading) return widget.whenReady;
+    return _delayPassed ? widget.whileDelayed : const SizedBox.shrink();
+  }
+}