chore: add nix flake dev shell

flake.nix

@@ -0,0 +1,107 @@
+{
+  description = "Bitkit Android dev shell (JDK 17 + Android SDK 36/NDK r28b via androidenv)";
+
+  # Provides the full toolchain to run: just compile / just build / just test / just lint
+  #
+  # Usage:
+  #   nix develop          # enter the shell
+  #   just compile         # or: ./gradlew assembleDevDebug
+  #
+  # Notes:
+  #   - First `nix develop` downloads ~2 GB of SDK/NDK; first gradle run
+  #     downloads the wrapper + all maven deps.
+  #   - Emulator/system images are excluded to keep the closure small.
+  #     For `just test android` without a physical device, flip the two
+  #     flags in androidenv below.
+  #   - On NixOS, adb access to a physical device needs udev rules:
+  #       services.udev.packages = [ pkgs.android-udev-rules ];
+  #   - A fully sandboxed `nix build` of the APK is not provided: gradle
+  #     needs network access for maven deps, which would require vendoring
+  #     the whole dependency graph (e.g. via gradle2nix).
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    { nixpkgs, ... }:
+    let
+      # Only x86_64-linux is verified; the rest should work via androidenv.
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in
+    {
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = import nixpkgs {
+            inherit system;
+            config = {
+              # Android SDK/NDK are unfree-licensed
+              allowUnfree = true;
+              android_sdk.accept_license = true;
+            };
+          };
+
+          buildToolsVersion = "36.0.0";
+          # Keep in sync with ndk_ver in Justfile
+          ndkVersion = "28.1.13356709";
+
+          androidSdk = pkgs.androidenv.composeAndroidPackages {
+            cmdLineToolsVersion = "latest";
+            platformToolsVersion = "latest";
+            platformVersions = [ "36" ]; # compileSdk/targetSdk
+            buildToolsVersions = [ buildToolsVersion ];
+            includeNDK = true;
+            ndkVersions = [ ndkVersion ];
+            includeEmulator = false;
+            includeSystemImages = false;
+            # No native code is built (rust libs ship as maven artifacts)
+            includeCmake = false;
+          };
+
+          sdkRoot = "${androidSdk.androidsdk}/libexec/android-sdk";
+        in
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              jdk17 # AGP 9.3 / Gradle 9.5 require JDK 17+
+              just
+              git
+            ];
+
+            JAVA_HOME = "${pkgs.jdk17.home}";
+            ANDROID_HOME = sdkRoot;
+            ANDROID_SDK_ROOT = sdkRoot;
+            ANDROID_NDK_ROOT = "${sdkRoot}/ndk/${ndkVersion}";
+
+            shellHook = ''
+              export PATH="$ANDROID_HOME/platform-tools:$PATH"
+
+              # AGP fetches aapt2 from maven as an unpatched ELF binary that
+              # cannot run on NixOS; point it at the patched one from the SDK.
+              aapt2="$ANDROID_HOME/build-tools/${buildToolsVersion}/aapt2"
+              gradle_props="''${GRADLE_USER_HOME:-$HOME/.gradle}/gradle.properties"
+              mkdir -p "$(dirname "$gradle_props")"
+              touch "$gradle_props"
+              if grep -qs '^android.aapt2FromMavenOverride=' "$gradle_props"; then
+                # Refresh overrides previously written by this hook (stale store
+                # paths break after a flake update + gc); leave custom ones alone.
+                sed -i "s|^android.aapt2FromMavenOverride=/nix/store/.*|android.aapt2FromMavenOverride=$aapt2|" "$gradle_props"
+              else
+                echo "android.aapt2FromMavenOverride=$aapt2" >> "$gradle_props"
+                echo ">> added aapt2 override to $gradle_props"
+              fi
+
+              echo "bitkit-android dev shell"
+              echo "  java:         $(java -version 2>&1 | head -1)"
+              echo "  ANDROID_HOME: $ANDROID_HOME"
+            '';
+          };
+        }
+      );
+    };
+}