feat: Phase 2 — upgrade deps to null-safe, replace deprecated packages

- pubspec: bump all maintained deps to null-safe latest; flutter pub get resolves
- Remove 12 packages (0-ref or no null-safe): splashscreen, flappy_search_bar,
  searchable_dropdown, share, platform_detect, flutter_inappwebview, countdown,
  gender_selection, flutter_dash, smooth_star_rating, hovering, ffi
- Add flutter_rating_bar
- Keep discontinued-but-null-safe for now (Phase 5): pull_to_refresh, hive, catcher
- New null-safe custom components (API-compatible, callers change only imports):
  utils/countdown.dart, widgets/general/{hover_widget,gender_selection,
  smooth_star_rating,search_bar}.dart
- main.dart: drop dead SplashScreen _buildBody + _to field
- 23 consuming files: swap deprecated imports to local components

Null-safety migration + upgraded-API adaptation = Phase 3.

UPGRADE_NOTES.md

@@ -15,7 +15,7 @@
 ## 阶段
 - [x] 阶段 0：基线 & 锁版本
 - [x] 阶段 1：去掉 Stripe
-- [ ] 阶段 2：依赖替换与升级
+- [x] 阶段 2：依赖替换与升级
 - [ ] 阶段 3：空安全迁移
 - [ ] 阶段 4：Web 跑起来
 
@@ -38,3 +38,18 @@
 - constants.dart: 新增 ETRANSFER_EMAIL(payment@wisetronic.com) 与 REMARK_PREFIX(R#)
 - 新增 lib/widgets/general/e_transfer_pay.dart: 展示邮箱 + 提醒备注写 R#<orderId>
 - 保留: PAYMENT_METHOD_CODE_STRIPE='stripe'(API 仍返回该 code，路由到 e-Transfer)；payment_platform.dart 的 'stripe' 显示名(后续美化时调整)
+
+### 阶段 2 — 依赖替换与升级（已完成）
+**flutter pub get 成功解析 178 个依赖。**
+- 升级到 null-safe 最新版：dio ^5.7、cached_network_image ^3.4、url_launcher ^6.3、path_provider ^2.1、image_picker ^1.1、google_maps_flutter ^2.9、geolocator ^12、youtube_player_iframe ^5.2、flutter_local_notifications ^17、uuid ^4、fluttertoast ^8.2、badges ^3、carousel_slider ^5、percent_indicator ^4.2.3、photo_view ^0.15、toggle_switch ^2.3、pinput ^5、email_validator ^3、responsive_builder ^0.7、flutter_redux ^0.10、fluro ^2.0.5、font_awesome_flutter ^10、flutter_spinkit ^5.2、flutter_markdown ^0.7、url_strategy ^0.3、universal_io/html
+- 删除(0引用或无 null-safe 版)：splashscreen(死代码)、flappy_search_bar、searchable_dropdown、share、platform_detect、flutter_inappwebview、countdown、gender_selection、flutter_dash(死 import)、smooth_star_rating、hovering、ffi
+- 新增：flutter_rating_bar ^4.0
+- 停维但有 null-safe 版→暂留(Phase 5 再换)：pull_to_refresh(→smart_refresher)、hive/hive_flutter(→hive_ce)、catcher ^0.8(→catcher2)
+- 新建 null-safe 自定义组件替代废弃包(API 兼容, 调用方仅改 import)：
+  - lib/utils/countdown.dart (CountDown.stream)
+  - lib/widgets/general/hover_widget.dart (HoverWidget)
+  - lib/widgets/general/gender_selection.dart (Gender 枚举 + GenderSelection)
+  - lib/widgets/general/smooth_star_rating.dart (SmoothStarRating)
+  - lib/widgets/general/search_bar.dart (轻量 Web 搜索 SearchBar + SearchBarController, 含 replayLastSearch 分页)
+- main.dart：删除未使用的 _buildBody(SplashScreen) 与 _to 字段
+- ⚠️ 待 Phase 3：API 升级破坏性变更(dio5/badges3/toggle_switch2/pinput5/geolocator12 等用法适配) + 整库空安全迁移，按 dart analyze 驱动修复