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.

lib/widgets/general/hover_widget.dart

@@ -0,0 +1,46 @@
+import 'package:flutter/material.dart';
+
+/// 自定义实现，替代已停维的 `hovering` 包的 [HoverWidget]。
+///
+/// API 兼容：
+/// ```dart
+/// HoverWidget(
+///   child: ...,
+///   hoverChild: ...,
+///   onHover: (PointerHoverEvent event) { ... },
+/// )
+/// ```
+///
+/// 鼠标悬停时显示 [hoverChild]，否则显示 [child]。
+class HoverWidget extends StatefulWidget {
+  final Widget child;
+  final Widget? hoverChild;
+  final void Function(PointerHoverEvent)? onHover;
+
+  const HoverWidget({
+    super.key,
+    required this.child,
+    this.hoverChild,
+    this.onHover,
+  });
+
+  @override
+  State<HoverWidget> createState() => _HoverWidgetState();
+}
+
+class _HoverWidgetState extends State<HoverWidget> {
+  bool _hovering = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return MouseRegion(
+      cursor: SystemMouseCursors.click,
+      onHover: widget.onHover,
+      onEnter: (_) => setState(() => _hovering = true),
+      onExit: (_) => setState(() => _hovering = false),
+      child: (_hovering && widget.hoverChild != null)
+          ? widget.hoverChild!
+          : widget.child,
+    );
+  }
+}