Add app source and tests

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

lib/Peach/designs/Backdrop.dart

@@ -0,0 +1,59 @@
+import 'package:flutter/material.dart';
+
+import '../utils/Colors.dart';
+
+/// App-wide ambient backdrop: a solid base with soft, blurred brand-tinted
+/// glows sitting behind the (transparent) scaffolds. Deliberately restrained —
+/// the warm vermilion anchors the brand, a cool counter-tone adds depth.
+class Backdrop extends StatelessWidget {
+  final Widget child;
+  const Backdrop({super.key, required this.child});
+
+  @override
+  Widget build(BuildContext context) {
+    final jp = context.jp;
+    final dark = Theme.of(context).brightness == Brightness.dark;
+    return DecoratedBox(
+      decoration: BoxDecoration(color: jp.bg),
+      child: Stack(
+        children: [
+          // Warm brand wash, upper-left.
+          Positioned(
+            top: -220,
+            left: -180,
+            child: _glow(PeachTheme.seed, dark ? 0.10 : 0.045, 560),
+          ),
+          // Cool counter-tone, lower-right, for depth.
+          Positioned(
+            bottom: -240,
+            right: -190,
+            child: _glow(PeachTheme.violet, dark ? 0.10 : 0.04, 560),
+          ),
+          // Faint high accent to lift the top of the canvas.
+          Positioned(
+            top: -320,
+            right: 40,
+            child: _glow(PeachTheme.accent, dark ? 0.05 : 0.02, 420),
+          ),
+          Positioned.fill(child: child),
+        ],
+      ),
+    );
+  }
+
+  Widget _glow(Color c, double opacity, double size) => IgnorePointer(
+    child: Container(
+      width: size,
+      height: size,
+      decoration: BoxDecoration(
+        shape: BoxShape.circle,
+        gradient: RadialGradient(
+          colors: [
+            c.withValues(alpha: opacity),
+            c.withValues(alpha: 0),
+          ],
+        ),
+      ),
+    ),
+  );
+}