perf(ui): stop the nav-rail brand pulse to fix web scroll jank

The DeltaMark ran an endless 9s breathing animation (60fps repaint)
inside the persistent nav rail, so it repainted on every scroll
frame and stuttered scrolling on Flutter web. Make the rail mark
static (animated: false) and isolate the animated variant (landing
page) behind a RepaintBoundary.

Signed-off-by: flemming-it <sf@flemming.it>

app/lib/widgets/delta_mark.dart

@@ -46,17 +46,30 @@ class _DeltaMarkState extends State<DeltaMark>
 
   @override
   Widget build(BuildContext context) {
-    return AnimatedBuilder(
-      animation: _controller,
-      builder: (context, _) {
-        final t = _controller.value;
-        // Breathe: scale 1 → 1.28 → 1 over the cycle.
-        final pulse = 1.0 + 0.28 * (1 - (math.cos(t * 2 * math.pi).abs()));
-        return CustomPaint(
-          size: Size.square(widget.size),
-          painter: _DeltaPainter(pulse: widget.animated ? pulse : 1.0),
-        );
-      },
+    // Static mark: no controller listening, paint once.
+    if (!widget.animated) {
+      return CustomPaint(
+        size: Size.square(widget.size),
+        painter: _DeltaPainter(pulse: 1.0),
+      );
+    }
+    // Animated: isolate the per-frame repaint in its own layer so
+    // the breathing pulse never invalidates surrounding content
+    // (web scroll stays smooth).
+    return RepaintBoundary(
+      child: AnimatedBuilder(
+        animation: _controller,
+        builder: (context, _) {
+          final t = _controller.value;
+          // Breathe: scale 1 → 1.28 → 1 over the cycle.
+          final pulse =
+              1.0 + 0.28 * (1 - (math.cos(t * 2 * math.pi).abs()));
+          return CustomPaint(
+            size: Size.square(widget.size),
+            painter: _DeltaPainter(pulse: pulse),
+          );
+        },
+      ),
     );
   }
 }