refactor: standardize Dart and Android config across all 54 apps

- Bump all apps to sdk >=3.0.0 <4.0.0 and add flutter_lints + a shared
  analysis_options.yaml to every app (flutter analyze is now green repo-wide)
- Fix real compile errors in firebase_google_authentication and image_editor,
  plus deprecated APIs (Matrix4, SvgPicture.color, textScaleFactor),
  missing async 'mounted' guards, and misc lints via dart fix + manual fixes
- Align pubspec 'name' with each folder and Android namespace/applicationId
  with the documented github.nisrulz.* convention
- Convert bmi_calculator from a legacy Flutter module to a standard app
  with a proper android/ scaffold; fix missing android:exported in launcher
  manifests across 22 apps so all Android builds pass
- Untrack generated build files (GeneratedPluginRegistrant,
  .flutter-plugins-dependencies) and ignore them

using_gradient/lib/main.dart

@@ -16,15 +16,15 @@ void main() {
           // Body
           body: Container(
               // Center the content
+              decoration: BoxDecoration(
+                  // Add Gradient
+                  gradient: utils.getCustomGradient()),
+              // Center the content
               child: Center(
                 // Add Text
                 child: Text(
                   "Hello World!",
                   style: TextStyle(color: Colors.white),
                 ),
-              ),
-              // Set background
-              decoration: BoxDecoration(
-                  // Add Gradient
-                  gradient: utils.getCustomGradient())))));
+              )))));
 }