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/build.gradle

@@ -7,10 +7,36 @@ allprojects {
 
 rootProject.buildDir = "../build"
 subprojects {
+    afterEvaluate { project ->
+        if (project.hasProperty("android")) {
+            project.android {
+                compileSdkVersion 36
+            }
+        }
+    }
     project.buildDir = "${rootProject.buildDir}/${project.name}"
-}
-subprojects {
-    project.evaluationDependsOn(":app")
+    project.evaluationDependsOn(':app')
+
+    plugins.withId('com.android.library') {
+        extensions.configure(com.android.build.gradle.LibraryExtension) { android ->
+            def manifestFile = android.sourceSets.main.manifest.srcFile
+            def packageName = null
+            if (manifestFile.exists()) {
+                def manifestContent = new XmlParser().parse(file(manifestFile))
+                packageName = manifestContent.@package
+            }
+            if (!android.hasProperty("namespace") || android.namespace == null || android.namespace.isEmpty()) {
+                if (packageName) {
+                    println ">> Derived Namespace for: '${project.name}' to '${packageName}'"
+                    android.namespace = packageName
+                } else {
+                    def defaultNamespace = "com.example.${project.name}"
+                    println "?? Made up Namespace for: '${project.name}' to '${defaultNamespace}'"
+                    android.namespace = defaultNamespace
+                }
+            }
+        }
+    }
 }
 
 tasks.register("clean", Delete) {