Android UX improvements: back button handling and debug branding

- Add BackHandler to AddFriendScreen (step navigation + dismiss)
- Add BackHandler to LocationSettingsScreen (dismiss to map)
- Add BackHandler to OnboardingScreen (page navigation)
- Add BackHandler to ShareBackScreen (dismiss to map)
- Add BackHandler to MapScreen edit mode (exit edit on back)
- Add debug build branding: "Coords Dev" name + red icon
- Add Android Studio to dev shell for interactive development
- Add proguard rule for errorprone annotations

.gitignore

···
 
 # Android
 android/.idea/
-android/.kotlin/errors
+android/.kotlin
 android/app/src/main/jniLibs/arm64-v8a/libtransponder_core.so
 
 # Apple build artifacts (regenerate with apple/build-xcframework.sh)

android/app/build.gradle.kts

···
     buildTypes {
         debug {
             applicationIdSuffix = ".dev"
+            resValue("string", "app_name", "Coords Dev")
             buildConfigField("Boolean", "USE_MOCK_FRIENDS", "true")
             // Temporarily test minification
             isMinifyEnabled = true
···
             )
         }
         release {
+            resValue("string", "app_name", "Coords")
             isMinifyEnabled = true
             isShrinkResources = true
             proguardFiles(

android/app/proguard-rules.pro

···
 # For more details, see
 #   http://developer.android.com/guide/developing/tools/proguard.html
 
+# AndroidX test references error-prone annotations not included at runtime
+-dontwarn com.google.errorprone.annotations.**
+
 # JNA - required for UniFFI native bindings
 # JNA uses reflection to find native methods
 -dontwarn java.awt.*

android/app/src/debug/res/drawable/ic_launcher_background.xml

···
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <solid android:color="#FF3B30"/>
+</shape>

android/app/src/main/java/sh/bentley/transponder/AddFriendScreen.kt

···
 import android.graphics.Bitmap
 import android.graphics.ImageFormat
 import android.util.Size
+import androidx.activity.compose.BackHandler
 import androidx.activity.compose.rememberLauncherForActivityResult
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.camera.core.CameraSelector
···
     var role by remember { mutableStateOf(ExchangeRole.SCAN_FIRST) }
     var currentStep by remember { mutableIntStateOf(0) }
     var addedFriendName by remember { mutableStateOf<String?>(null) }
+
+    BackHandler {
+        if (currentStep > 0) {
+            currentStep = 0
+            addedFriendName = null
+        } else {
+            onDismiss()
+        }
+    }
 
     Scaffold(
         topBar = {
···
     friendName: String,
     onDismiss: () -> Unit
 ) {
+    BackHandler { onDismiss() }
+
     val qrBitmap = remember(identityStore) {
         val identity = identityStore.getIdentity() ?: return@remember null
         val serverUrl = identityStore.serverUrl ?: return@remember null

android/app/src/main/java/sh/bentley/transponder/LocationSettingsScreen.kt

···
 package sh.bentley.transponder
 
+import androidx.activity.compose.BackHandler
 import androidx.compose.foundation.layout.*
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.Close
···
 ) {
     val context = LocalContext.current
     val scope = rememberCoroutineScope()
+
+    BackHandler { onDismiss() }
 
     // Settings state
     var activeTimeoutSec by remember { mutableIntStateOf((identityStore.locationActiveTimeoutMs / 1000).toInt()) }

android/app/src/main/java/sh/bentley/transponder/MapScreen.kt

···
     var isEditMode by remember { mutableStateOf(false) }
     var friendToDelete by remember { mutableStateOf<Friend?>(null) }
     var showDeleteConfirmation by remember { mutableStateOf(false) }
+
+    // Exit edit mode on back press
+    BackHandler(enabled = isEditMode) {
+        isEditMode = false
+    }
     var myName by remember { mutableStateOf(identityStore.displayName ?: "Me") }
 
     // Check if auto-share should be enabled (requires both setting AND permission)

android/app/src/main/java/sh/bentley/transponder/OnboardingScreen.kt

···
 import android.Manifest
 import android.content.pm.PackageManager
 import android.os.Build
+import androidx.activity.compose.BackHandler
 import androidx.activity.compose.rememberLauncherForActivityResult
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.compose.animation.AnimatedContent
···
     var currentPage by remember { mutableStateOf(OnboardingPage.Welcome) }
     var hasLocationPermission by remember { mutableStateOf(false) }
     var hasBackgroundPermission by remember { mutableStateOf(false) }
+
+    BackHandler(enabled = currentPage != OnboardingPage.Welcome) {
+        currentPage = when (currentPage) {
+            OnboardingPage.Identity -> OnboardingPage.Location
+            OnboardingPage.Location -> OnboardingPage.Welcome
+            OnboardingPage.Welcome -> OnboardingPage.Welcome
+        }
+    }
 
     Scaffold { innerPadding ->
         AnimatedContent(

flake.nix

···
         default = pkgs.mkShell {
           buildInputs = [
             android-toolchain
+            pkgs.android-studio
             pkgs.rust-analyzer
             pkgs.nodejs_24
             pkgs.git