devbox setup, dep upgrades, lint fixes, Android build fixes

- Add devbox.json pinning Flutter 3.35.7, Android SDK (Nix flake),
  openjdk17, just, openssl
- Add android/nix-android-sdk/ Nix flake + flake.lock + Android Studio
  config script
- Add justfile with build/get/upgrade/icons/analyze/rebuild recipes
- flutter pub upgrade --major-versions: file_picker ^11, google_fonts ^8,
  permission_handler ^12, share_plus ^12, flutter_lints ^6 (+71 transitive)
- Fix file_picker v11 API: FilePicker.platform → FilePicker.getDirectoryPath()
- Fix all 37 analyzer issues: deprecated APIs (window, WillPopScope, Share,
  SvgPicture color), async BuildContext guards, key constructors, private
  createState() return types, unused locals, print → debugPrint, etc.
- Hook _handleCommand into onSubmitted for : prefix commands
- Android: Gradle 8.3→8.11.1, AGP 8.2.2→8.9.1, KGP 1.8.22→2.2.20,
  compileSdk/targetSdk 34→36, namespace auto-derivation for library modules,
  kotlin.compiler.execution.strategy=in-process

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/main.dart

@@ -14,12 +14,14 @@ void main() async {
     final license = await rootBundle.loadString('google_fonts/OFL.txt');
     yield LicenseEntryWithLineBreaks(['google_fonts'], license);
   });
-  runApp(MuzzleVelocityApp());
+  runApp(const MuzzleVelocityApp());
 }
 
 class MuzzleVelocityApp extends StatefulWidget {
+  const MuzzleVelocityApp({super.key});
+
   @override
-  _MuzzleVelocityAppState createState() => _MuzzleVelocityAppState();
+  State<MuzzleVelocityApp> createState() => _MuzzleVelocityAppState();
 }
 
 class _MuzzleVelocityAppState extends State<MuzzleVelocityApp> {
@@ -31,7 +33,7 @@ class _MuzzleVelocityAppState extends State<MuzzleVelocityApp> {
     super.initState();
     // Ensure the correct theme mode is set on startup
     _themeMode =
-        WidgetsBinding.instance.window.platformBrightness == Brightness.dark
+        WidgetsBinding.instance.platformDispatcher.platformBrightness == Brightness.dark
             ? ThemeMode.dark
             : ThemeMode.light;
   }