feat(brand): app icon painter + generated 1024px icon PNGs

Introduces AppIconMark (navy gradient field + 2×2 glossy blocks reusing
paintGlossyTile) and a testWidgets generator that writes icon.png,
icon_background.png, and icon_foreground.png to assets/icon/ for the
upcoming flutter_launcher_icons task.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/branding/app_icon_painter.dart

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+// lib/ui/branding/app_icon_painter.dart
+import 'dart:ui';
+
+import 'package:flutter/material.dart';
+
+import '../widgets/tile_painter.dart';
+
+/// Draws the Block Seasons brand mark: deep-navy field + a 2×2 grid of glossy
+/// brand-color blocks. Shared by the launcher-icon and feature-graphic
+/// generators so the brand stays identical everywhere.
+class AppIconMark {
+  static const navy = [Color(0xFF101736), Color(0xFF192555), Color(0xFF2C3168)];
+  static const pink = Color(0xFFFF7EB3);
+  static const yellow = Color(0xFFFFD166);
+  static const cyan = Color(0xFF6FCDF5);
+  static const green = Color(0xFF7EDB9C);
+
+  /// Fills [rect] with the navy gradient.
+  static void paintBackground(Canvas canvas, Rect rect) {
+    final paint = Paint()
+      ..shader = const LinearGradient(
+        begin: Alignment.topLeft,
+        end: Alignment.bottomRight,
+        colors: navy,
+      ).createShader(rect);
+    canvas.drawRect(rect, paint);
+  }
+
+  /// Paints the 2×2 glossy blocks centered in a square of side [size], the
+  /// block group occupying [groupFraction] of the side.
+  static void paintBlocks(Canvas canvas, double size,
+      {double groupFraction = 0.6}) {
+    final group = size * groupFraction;
+    final gap = size * 0.05;
+    final block = (group - gap) / 2;
+    final m = (size - group) / 2;
+    final far = m + block + gap;
+    void tile(double x, double y, Color c) => paintGlossyTile(
+        canvas, Rect.fromLTWH(x, y, block, block), c, radiusFactor: 0.24);
+    tile(m, m, pink);
+    tile(far, m, yellow);
+    tile(m, far, cyan);
+    tile(far, far, green);
+  }
+}