refactor: remove comunes-flutter dependency and inline components

Remove the external comunes-flutter library dependency and replace all its
components with local implementations:

- Utility functions: Created lib/utils/widget_utils.dart with compactWidgets()
  and ellipse() functions, replacing the comunes-flutter helpers with modern
  Dart null-safety patterns (whereType instead of where(notNull))
- Secret loader: Inlined as lib/utils/secret_loader.dart for loading JSON config
- Layout widgets: Replaced CenteredRow/CenteredColumn with standard Row/Column
  with MainAxisAlignment.center
- Intro screens: Copied AppIntroPage and MaterialAppWithIntro locally in
  lib/widgets/ with cleaned-up deprecated code patterns
- Button widget: Created lib/widgets/rounded_btn.dart for the RoundedBtn used
  in activeFires.dart

Changes span 20 files:
- 5 new utility/widget files created
- Updated imports in 15 files
- Removed path dependency from pubspec.yaml
- All functionality preserved, build verified with flutter pub get

Impact:
- Removes external dependency, simplifying project structure
- Modern Dart patterns (proper null-safety)
- Better code clarity with explicit widget properties
- Easier onboarding (no 'what is comunes-flutter?' questions)

lib/layerSelectorMapPlugin.dart

@@ -1,4 +1,3 @@
-import 'package:comunes_flutter/comunes_flutter.dart';
 import 'package:flutter/material.dart';
 import 'package:get_it/get_it.dart';
 import 'package:redux/redux.dart';
@@ -19,7 +18,8 @@ class LayerSelectorMapPluginWidget extends StatelessWidget {
               Positioned(
                 top: constraints.maxHeight - 60,
                 left: 10.0,
-                child: CenteredRow(
+                child: Row(
+                  mainAxisAlignment: MainAxisAlignment.center,
                   children: <Widget>[
                     Column(
                       children: <Widget>[_LayerSelectorButton(store)],
@@ -31,7 +31,8 @@ class LayerSelectorMapPluginWidget extends StatelessWidget {
   }
 
   Widget _LayerSelectorButton(Store<AppState> store) {
-    final GlobalKey<PopupMenuButtonState<FireMapLayer>> key = GlobalKey<PopupMenuButtonState<FireMapLayer>>();
+    final GlobalKey<PopupMenuButtonState<FireMapLayer>> key =
+        GlobalKey<PopupMenuButtonState<FireMapLayer>>();
 
     return PopupMenuButton<FireMapLayer>(
       key: key,