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>

android/app/build.gradle

@@ -6,10 +6,10 @@ plugins {
 }
 
 android {
-    namespace = "com.example.muzzlevelocity"
+    namespace = "vision.flux.muzzlevelocity"
     compileSdk = flutter.compileSdkVersion
     ndkVersion = flutter.ndkVersion
-    compileSdkVersion 34
+    compileSdkVersion 36
 
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_1_8
@@ -22,14 +22,14 @@ android {
 
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
-        applicationId = "com.example.muzzlevelocity"
+        applicationId = "vision.flux.muzzlevelocity"
         // You can update the following values to match your application needs.
         // For more information, see: https://flutter.dev/to/review-gradle-config.
         minSdk = flutter.minSdkVersion
         targetSdk = flutter.targetSdkVersion
         versionCode = flutter.versionCode
         versionName = flutter.versionName
-        targetSdkVersion 34
+        targetSdkVersion 36
     }
 
     buildTypes {