Android: fix OS back gesture for SPA modals/screens (drive JS history.back)

WryActivity's generated back handler calls WebView.goBack(), which does NOT
dispatch popstate for same-document history.pushState() entries — so the OS back
gesture never reached the app's popstate handler and couldn't dismiss modals
(new product, add-to-eaten, ЧЗ scanner) or navigate screens; only the on-screen
back arrow worked. WryActivity.kt is auto-regenerated each build, so override in
the app-owned MainActivity instead: set handleBackNavigation=false and register
our own OnBackPressedCallback that runs window.history.back() via evaluateJavascript,
which fires popstate. Verified on-device: swipe-back now closes modals and the ЧЗ
scanner. (adb `input keyevent BACK` does NOT go through OnBackPressedDispatcher on
Android 13+, so it can't be used to test this — only the real gesture does.)

src-tauri/gen/android/app/src/main/java/dev/baldav/app/MainActivity.kt

@@ -11,6 +11,8 @@ import android.provider.Settings
 import android.os.Bundle
 import android.os.FileObserver
 import android.util.Log
+import android.webkit.WebView
+import androidx.activity.OnBackPressedCallback
 import androidx.activity.enableEdgeToEdge
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.core.view.ViewCompat
@@ -22,6 +24,33 @@ import javax.net.ssl.HttpsURLConnection
 class MainActivity : TauriActivity() {
   private var commandObserver: FileObserver? = null
   private var pendingSaveContent: String? = null
+  private var backWebView: WebView? = null
+
+  // Disable WryActivity's generated back handler: its mWebView.goBack() does NOT
+  // dispatch a popstate for same-document history.pushState() entries (our SPA
+  // nav/modals), so the OS back gesture never reached the JS handler — modals
+  // and screens couldn't be dismissed with a swipe. We register our own callback
+  // (below) that drives window.history.back() via JS, which fires popstate.
+  override val handleBackNavigation: Boolean = false
+
+  override fun onWebViewCreate(webView: WebView) {
+    super.onWebViewCreate(webView)
+    backWebView = webView
+    onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
+      override fun handleOnBackPressed() {
+        val wv = backWebView
+        if (wv != null) {
+          // Drive JS history.back() — it fires popstate (closes modals / navigates
+          // screens). WebView.goBack() would not, for same-document pushState entries.
+          wv.evaluateJavascript("window.history.back()", null)
+        } else {
+          isEnabled = false
+          onBackPressed()
+          isEnabled = true
+        }
+      }
+    })
+  }
 
   private val saveFileLauncher = registerForActivityResult(
     ActivityResultContracts.CreateDocument("application/gpx+xml")