Add a sourceable Android env helper and document the Studio workflow

scripts/android-env.sh exports ANDROID_HOME/NDK_HOME/JAVA_HOME by
auto-detection, so there are no machine-specific paths to keep in sync. It
skips prerelease NDKs when several are installed and prefers Android Studio's
bundled JDK 21, which Gradle accepts.

Also document that Android Studio must be launched via
`cargo tauri android dev --open` rather than by opening src-tauri/gen/android
directly: Studio's Run button delegates the Rust build to
`tauri android android-studio-script`, which reads the dev-server address from
/tmp/<identifier>-server-addr and fails with "missing addr file" if no Tauri
dev session is running. `cargo tauri android build` needs no addr file.

README.md

@@ -141,16 +141,39 @@ prints the three environment variables to export (`ANDROID_HOME`, `NDK_HOME`,
 
 ### Build and run
 
+Every Android command needs `ANDROID_HOME` / `NDK_HOME` / `JAVA_HOME`. Source
+the helper once per shell — it auto-detects all three:
+
 ```bash
-cargo tauri android init      # once — scaffolds src-tauri/gen/android
-cargo tauri android dev       # run on an emulator or adb-connected device
-cargo tauri android build --apk
+source scripts/android-env.sh
+```
+
+Then, from the project root:
+
+```bash
+cargo tauri android dev --open   # dev server + open in Android Studio
+cargo tauri android build --apk  # standalone release APK
 ```
 
 `src-tauri/gen/android` is committed (unlike the rest of `gen/`) because
 `AndroidManifest.xml` is hand-edited — Android permissions have no
 `tauri.conf.json` equivalent.
 
+### Working in Android Studio
+
+Use **`cargo tauri android dev --open`** rather than opening
+`src-tauri/gen/android` directly. Studio's Run button delegates the Rust build
+to `cargo tauri android android-studio-script`, which expects a running Tauri
+dev session — it reads the dev-server address from
+`/tmp/<identifier>-server-addr`. Open the Gradle project on its own and the
+build fails with `failed to read missing addr file`.
+
+With the dev session running, Studio's device dropdown, Device Manager
+(emulator), and debugger all work normally.
+
+To build without Studio and without a dev server, use `cargo tauri android
+build` — that path needs no addr file.
+
 ### Card data on a phone
 
 Downloading card data streams ~740MB of JSON to build a ~73MB database, which

scripts/android-env.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+#
+# android-env.sh — export the environment the Android build needs.
+#
+# SOURCE this, don't execute it:
+#
+#   source scripts/android-env.sh
+#   cargo tauri android dev --open
+#
+# Everything is auto-detected, so there are no machine-specific paths to keep in
+# sync. Run ./scripts/android-setup.sh first to install the pieces.
+
+_mtgc_android_env() {
+    local sdk ndk jdk
+
+    # --- SDK ---
+    sdk="${ANDROID_HOME:-$HOME/Android/Sdk}"
+    if [ ! -d "$sdk" ]; then
+        echo "android-env: no Android SDK at $sdk (run ./scripts/android-setup.sh)" >&2
+        return 1
+    fi
+
+    # --- NDK: newest *stable* installed (skip beta/rc/canary) ---
+    local best=""
+    for d in "$sdk"/ndk/*; do
+        [ -d "$d" ] || continue
+        case "$(sed -n 's/^Pkg.Revision *= *//p' "$d/source.properties" 2>/dev/null)" in
+            *beta*|*rc*|*canary*) continue ;;
+        esac
+        best="$d"
+    done
+    ndk="$best"
+    if [ -z "$ndk" ]; then
+        echo "android-env: no stable NDK under $sdk/ndk (run ./scripts/android-setup.sh)" >&2
+        return 1
+    fi
+
+    # --- JDK: Gradle/AGP reject very new JDKs; prefer Studio's bundled JBR 21 ---
+    for d in \
+        "$HOME/.local/share/JetBrains/Toolbox/apps/android-studio/jbr" \
+        "$HOME/.local/share/JetBrains/Toolbox/apps/android-studio"/*/jbr \
+        "$HOME/android-studio/jbr" \
+        /opt/android-studio/jbr \
+        /usr/lib/jvm/java-21-openjdk /usr/lib/jvm/java-21-openjdk-* \
+        /usr/lib/jvm/java-17-openjdk /usr/lib/jvm/java-17-openjdk-*
+    do
+        if [ -x "$d/bin/javac" ]; then jdk="$d"; break; fi
+    done
+    if [ -z "$jdk" ]; then
+        echo "android-env: no JDK 21/17 found (Gradle rejects the system default)" >&2
+        return 1
+    fi
+
+    export ANDROID_HOME="$sdk"
+    export NDK_HOME="$ndk"
+    export JAVA_HOME="$jdk"
+    # adb/emulator live here; harmless if already present.
+    case ":$PATH:" in
+        *":$sdk/platform-tools:"*) ;;
+        *) export PATH="$sdk/platform-tools:$PATH" ;;
+    esac
+
+    echo "ANDROID_HOME=$ANDROID_HOME"
+    echo "NDK_HOME=$NDK_HOME"
+    echo "JAVA_HOME=$JAVA_HOME"
+}
+
+_mtgc_android_env
+unset -f _mtgc_android_env