feat(macos): сборка DMG, фикс keychain и opus-библиотек

- secure storage: usesDataProtectionKeychain=false — вход в аккаунт
  больше не падает с -34018 без аккаунта разработчика
- entitlements: убран app-sandbox/keychain-access-groups (adhoc без Team ID)
- иконка: squircle с прозрачными полями (нативный macOS-вид)
- opus/ogg: битые bitcode-библиотеки ogg_opus_player заменены на
  собранные из исходников universal (.a в macos/prebuilt/opus +
  scripts/fix_opus_macos.sh применяет их после pub get)
- scripts/build_dmg.sh: adhoc-сборка universal DMG с дизайном (create-dmg)
- CI: build-macos.yml собирает DMG-артефакт; flutter-main.yml чинит libs

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tool/make_dmg_background.dart

@@ -0,0 +1,32 @@
+import 'dart:io';
+import 'package:image/image.dart' as img;
+
+void main() {
+  const w = 1280;
+  const h = 800;
+  final bg = img.Image(width: w, height: h, numChannels: 4);
+
+  final topR = 251, topG = 251, topB = 253;
+  final botR = 233, botG = 233, botB = 237;
+  for (var y = 0; y < h; y++) {
+    final t = y / (h - 1);
+    final r = (topR + (botR - topR) * t).round();
+    final g = (topG + (botG - topG) * t).round();
+    final b = (topB + (botB - topB) * t).round();
+    img.fillRect(bg, x1: 0, y1: y, x2: w - 1, y2: y, color: img.ColorRgb8(r, g, b));
+  }
+
+  final green = img.ColorRgb8(124, 191, 64);
+  const ay = 380;
+  img.fillRect(bg, x1: 548, y1: ay - 9, x2: 704, y2: ay + 9, color: green);
+  img.fillPolygon(bg, vertices: [
+    img.Point(700, ay - 30),
+    img.Point(700, ay + 30),
+    img.Point(752, ay),
+  ], color: green);
+
+  final outDir = Directory('scripts/dmg');
+  outDir.createSync(recursive: true);
+  File('scripts/dmg/background.png').writeAsBytesSync(img.encodePng(bg));
+  stdout.writeln('wrote scripts/dmg/background.png (${w}x$h)');
+}

tool/make_macos_icon.dart

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+import 'dart:io';
+import 'dart:math' as math;
+import 'package:image/image.dart' as img;
+
+const _dir = 'macos/Runner/Assets.xcassets/AppIcon.appiconset';
+
+const _sizes = {
+  'app_icon_16.png': 16,
+  'app_icon_32.png': 32,
+  'app_icon_64.png': 64,
+  'app_icon_128.png': 128,
+  'app_icon_256.png': 256,
+  'app_icon_512.png': 512,
+  'app_icon_1024.png': 1024,
+};
+
+void main() {
+  final source = img.decodePng(File('assets/meteor_icon.png').readAsBytesSync()) ??
+      img.decodePng(File('$_dir/app_icon_1024.png').readAsBytesSync())!;
+
+  const canvas = 1024;
+  const content = 824;
+  const margin = (canvas - content) / 2;
+  const radius = 184.0;
+
+  final body = img.copyResize(source,
+      width: content, height: content, interpolation: img.Interpolation.cubic);
+
+  final master = img.Image(width: canvas, height: canvas, numChannels: 4);
+  img.compositeImage(master, body, dstX: margin.toInt(), dstY: margin.toInt());
+
+  _applyRoundedMask(master, margin, margin, margin + content, margin + content, radius);
+
+  for (final entry in _sizes.entries) {
+    final out = img.copyResize(master,
+        width: entry.value,
+        height: entry.value,
+        interpolation: img.Interpolation.cubic);
+    File('$_dir/${entry.key}').writeAsBytesSync(img.encodePng(out));
+    stdout.writeln('wrote ${entry.key} (${entry.value}x${entry.value})');
+  }
+}
+
+void _applyRoundedMask(
+    img.Image im, double x0, double y0, double x1, double y1, double r) {
+  final cx = (x0 + x1) / 2;
+  final cy = (y0 + y1) / 2;
+  final hw = (x1 - x0) / 2;
+  final hh = (y1 - y0) / 2;
+  for (final p in im) {
+    final qx = (p.x + 0.5 - cx).abs() - (hw - r);
+    final qy = (p.y + 0.5 - cy).abs() - (hh - r);
+    final dx = math.max(qx, 0.0);
+    final dy = math.max(qy, 0.0);
+    final dist =
+        math.sqrt(dx * dx + dy * dy) + math.min(math.max(qx, qy), 0.0) - r;
+    final coverage = (0.5 - dist).clamp(0.0, 1.0);
+    if (coverage < 1.0) {
+      p.a = (p.a * coverage).round().clamp(0, 255);
+    }
+  }
+}