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.

linux/flutter/generated_plugin_registrant.cc

@@ -7,12 +7,16 @@
 #include "generated_plugin_registrant.h"
 
 #include <catcher/catcher_plugin.h>
+#include <file_selector_linux/file_selector_plugin.h>
 #include <url_launcher_linux/url_launcher_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
   g_autoptr(FlPluginRegistrar) catcher_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "CatcherPlugin");
   catcher_plugin_register_with_registrar(catcher_registrar);
+  g_autoptr(FlPluginRegistrar) file_selector_linux_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "FileSelectorPlugin");
+  file_selector_plugin_register_with_registrar(file_selector_linux_registrar);
   g_autoptr(FlPluginRegistrar) url_launcher_linux_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "UrlLauncherPlugin");
   url_launcher_plugin_register_with_registrar(url_launcher_linux_registrar);

linux/flutter/generated_plugins.cmake

@@ -4,6 +4,7 @@
 
 list(APPEND FLUTTER_PLUGIN_LIST
   catcher
+  file_selector_linux
   url_launcher_linux
 )