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>

pubspec.yaml

@@ -10,26 +10,27 @@ dependencies:
   flutter:
     sdk: flutter
   cupertino_icons: ^1.0.8
-  file_picker: ^9.0.0
+  file_picker: ^11.0.2
   flutter_highlight: ^0.7.0
   flutter_code_editor: ^0.3.2
   flutter_markdown: ^0.7.6+2
   flutter_markdown_latex: ^0.3.4
-  google_fonts: ^6.2.1
+  google_fonts: ^8.1.0
   markdown: ^7.3.0
   path: ^1.9.0
   path_provider: ^2.1.5
-  permission_handler: ^11.4.0
+  permission_handler: ^12.0.3
   shared_preferences: ^2.5.2
   highlight: ^0.7.0
   open_filex: ^4.6.0
   flutter_svg: ^2.0.17
   flutter_launcher_icons: ^0.14.3
+  share_plus: ^12.0.2
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
-  flutter_lints: ^5.0.0
+  flutter_lints: ^6.0.0
 
 flutter:
   uses-material-design: true