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/gender_selection.dart

@@ -0,0 +1,111 @@
+import 'package:flutter/material.dart';
+
+/// 自定义实现，替代已停维的 `gender_selection` 包。
+///
+/// 保留原 `Gender` 枚举与 `GenderSelection` 组件的命名参数，
+/// 使调用方仅改 import 即可编译。大部分装饰性参数为兼容保留（暂未全部生效，
+/// 后续美化阶段 Phase 5 再完善视觉）。
+enum Gender { Male, Female }
+
+class GenderSelection extends StatefulWidget {
+  final String? maleText;
+  final String? femaleText;
+  final Color? selectedGenderIconBackgroundColor;
+  final Alignment? checkIconAlignment;
+  final IconData? selectedGenderCheckIcon;
+  final void Function(Gender gender)? onChanged;
+  final bool? equallyAligned;
+  final Duration? animationDuration;
+  final bool? isCircular;
+  final bool? isSelectedGenderIconCircular;
+  final double? opacityOfGradient;
+  final EdgeInsets? padding;
+  final double? size;
+  final Gender? selectedGender;
+
+  const GenderSelection({
+    super.key,
+    this.maleText,
+    this.femaleText,
+    this.selectedGenderIconBackgroundColor,
+    this.checkIconAlignment,
+    this.selectedGenderCheckIcon,
+    this.onChanged,
+    this.equallyAligned,
+    this.animationDuration,
+    this.isCircular,
+    this.isSelectedGenderIconCircular,
+    this.opacityOfGradient,
+    this.padding,
+    this.size,
+    this.selectedGender,
+  });
+
+  @override
+  State<GenderSelection> createState() => _GenderSelectionState();
+}
+
+class _GenderSelectionState extends State<GenderSelection> {
+  Gender? _selected;
+
+  @override
+  void initState() {
+    super.initState();
+    _selected = widget.selectedGender;
+  }
+
+  Widget _buildOption(Gender gender, String label) {
+    final selected = _selected == gender;
+    final color = widget.selectedGenderIconBackgroundColor ?? Colors.indigo;
+    return Expanded(
+      child: Padding(
+        padding: widget.padding ?? const EdgeInsets.all(3.0),
+        child: InkWell(
+          onTap: () {
+            setState(() => _selected = gender);
+            widget.onChanged?.call(gender);
+          },
+          child: Container(
+            height: widget.size ?? 50.0,
+            alignment: Alignment.center,
+            decoration: BoxDecoration(
+              color: selected ? color : color.withOpacity(0.06),
+              borderRadius:
+                  (widget.isCircular ?? false) ? BorderRadius.circular(60) : null,
+              border: Border.all(
+                color: selected ? color : Colors.black26,
+                width: selected ? 2 : 1,
+              ),
+            ),
+            child: Row(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                if (selected && widget.selectedGenderCheckIcon != null)
+                  Icon(widget.selectedGenderCheckIcon, color: Colors.white, size: 18),
+                if (selected && widget.selectedGenderCheckIcon != null)
+                  const SizedBox(width: 6),
+                Text(
+                  label,
+                  style: TextStyle(
+                    color: selected ? Colors.white : Colors.black87,
+                    fontWeight: FontWeight.w600,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      children: <Widget>[
+        _buildOption(Gender.Male, widget.maleText ?? 'Mr'),
+        _buildOption(Gender.Female, widget.femaleText ?? 'Ms'),
+      ],
+    );
+  }
+}