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/nix-android-sdk/flake.nix

@@ -0,0 +1,27 @@
+{
+  description = "A Flake for the Android SDK";
+
+  inputs = {
+    flake-utils.url = "github:numtide/flake-utils";
+    android-nixpkgs.url = "github:tadfisher/android-nixpkgs/stable";
+  };
+
+  outputs = { self, flake-utils, android-nixpkgs }:
+    flake-utils.lib.eachSystem ["x86_64-linux" "x86_64-darwin" "aarch64-darwin"] (system: {
+    packages.android-sdk = android-nixpkgs.sdk."${system}" (sdkPkgs: with sdkPkgs; [
+      cmdline-tools-latest
+      build-tools-33-0-0
+      build-tools-34-0-0
+      build-tools-35-0-0
+      build-tools-36-1-0
+      cmake-3-22-1
+      platform-tools
+      platforms-android-33
+      platforms-android-34
+      platforms-android-35
+      platforms-android-36
+      emulator
+      ndk-27-0-12077973
+    ]);
+  });
+}