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/pages/shell_page.dart

@@ -50,7 +50,11 @@ class _ShellPageState extends State<ShellPage> {
               ),
               child: Column(
                 children: [
-                  const DeltaMark(size: 36),
+                  // Static in the persistent nav rail: a 60fps pulse
+                  // here repaints every scroll frame and stutters web
+                  // scrolling. The breathing mark stays on the
+                  // landing page (which isn't scrolled).
+                  const DeltaMark(size: 36, animated: false),
                   const SizedBox(height: ReclaimSpace.sm),
                   const BrandWordmark(size: 18),
                   Text(

app/lib/widgets/delta_mark.dart

@@ -46,17 +46,30 @@ class _DeltaMarkState extends State<DeltaMark>
 
   @override
   Widget build(BuildContext context) {
-    return AnimatedBuilder(
+    // 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()));
+          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),
+            painter: _DeltaPainter(pulse: pulse),
           );
         },
+      ),
     );
   }
 }