perf(assets): compress oversized images to reduce load and bundle size

- Shrink heavy backgrounds and icon source assets
- Reduce login background from ~8.1MB to ~0.43MB
- Add reusable tool script for repeatable asset optimization

tool/optimize_assets.dart

@@ -0,0 +1,75 @@
+import 'dart:io';
+import 'package:image/image.dart' as img;
+
+class _AssetRule {
+  final String path;
+  final int maxDimension;
+  final int jpgQuality;
+
+  const _AssetRule(this.path, {required this.maxDimension, this.jpgQuality = 86});
+}
+
+const _rules = <_AssetRule>[
+  _AssetRule('assets/img/login_bg.jpg', maxDimension: 1920, jpgQuality: 84),
+  _AssetRule('assets/img/loadingScreenMainScreen.png', maxDimension: 1920),
+  _AssetRule('assets/img/icon.png', maxDimension: 1024),
+  _AssetRule('assets/img/icon_bg_removed.png', maxDimension: 1024),
+  _AssetRule('assets/icon/app_icon.png', maxDimension: 1024),
+  _AssetRule('assets/icon/app_icon_ios.png', maxDimension: 1024),
+  _AssetRule('assets/icon/app_icon_foreground.png', maxDimension: 1024),
+];
+
+void main() {
+  for (final rule in _rules) {
+    final file = File(rule.path);
+    if (!file.existsSync()) {
+      stdout.writeln('SKIP  ${rule.path} (missing)');
+      continue;
+    }
+
+    final beforeBytes = file.lengthSync();
+    final decoded = img.decodeImage(file.readAsBytesSync());
+    if (decoded == null) {
+      stdout.writeln('SKIP  ${rule.path} (decode failed)');
+      continue;
+    }
+
+    img.Image working = decoded;
+    final maxSide = working.width > working.height ? working.width : working.height;
+
+    if (maxSide > rule.maxDimension) {
+      final scale = rule.maxDimension / maxSide;
+      final newWidth = (working.width * scale).round();
+      final newHeight = (working.height * scale).round();
+      working = img.copyResize(
+        working,
+        width: newWidth,
+        height: newHeight,
+        interpolation: img.Interpolation.average,
+      );
+    }
+
+    final lower = rule.path.toLowerCase();
+    late final List<int> encoded;
+    if (lower.endsWith('.jpg') || lower.endsWith('.jpeg')) {
+      encoded = img.encodeJpg(working, quality: rule.jpgQuality);
+    } else if (lower.endsWith('.png')) {
+      encoded = img.encodePng(working, level: 6);
+    } else {
+      stdout.writeln('SKIP  ${rule.path} (unsupported type)');
+      continue;
+    }
+
+    file.writeAsBytesSync(encoded, flush: true);
+    final afterBytes = file.lengthSync();
+    final saved = beforeBytes - afterBytes;
+    final pct = beforeBytes == 0 ? 0 : (saved / beforeBytes * 100);
+
+    stdout.writeln(
+      'DONE  ${rule.path}  '
+      '${(beforeBytes / 1024).toStringAsFixed(1)}KB -> '
+      '${(afterBytes / 1024).toStringAsFixed(1)}KB '
+      '(${pct >= 0 ? '-' : '+'}${pct.abs().toStringAsFixed(1)}%)',
+    );
+  }
+}