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/utils/widget_utils.dart

@@ -0,0 +1,13 @@
+import 'package:flutter/material.dart';
+
+/// Modern null-safe widget list compaction.
+/// Filters out null values from a list of nullable widgets.
+/// Example: compactWidgets([widget1, null, widget2]) → [widget1, widget2]
+List<Widget> compactWidgets(List<Widget?> children) =>
+    children.whereType<Widget>().toList();
+
+/// String truncation for debug output and logging.
+/// Truncates a string to [end] characters and appends '...'
+/// Example: ellipse('abcdefgh', 4) → 'abcd...'
+String ellipse(String s, [int end = 4]) =>
+    s.length > end ? '${s.substring(0, end)}...' : s;