fixed client-reported crashes in the pdf viewer, image picker and prosemirror images, mapped webdav errors to user messages and hardened keychain, cache and push stores against io failures

lib/widget/file_pick.dart

@@ -53,13 +53,21 @@ class FilePick {
     return result?.files.map((e) => e.path).nonNulls.toList();
   }
 
+  static bool _inFlight = false;
+
+  /// A second tap while the native picker is already open (double tap, or a
+  /// tap on another source tile) would make the plugin throw instead of
+  /// opening a second picker; it is treated as a cancel.
   static Future<T?> _guarded<T>(
     BuildContext context,
     Future<T?> Function() pick,
   ) async {
+    if (_inFlight) return null;
+    _inFlight = true;
     try {
       return await pick();
     } on PlatformException catch (e) {
+      if (_isAlreadyActive(e)) return null;
       final denied = PermissionException.fromPlatform(e);
       if (!context.mounted) return null;
       if (denied != null) {
@@ -71,9 +79,15 @@ class FilePick {
     } catch (e) {
       if (context.mounted) _showErrorDialog(context, e);
       return null;
+    } finally {
+      _inFlight = false;
     }
   }
 
+  // Android reports `already_active`, iOS `multiple_request`.
+  static bool _isAlreadyActive(PlatformException e) =>
+      e.code == 'already_active' || e.code == 'multiple_request';
+
   static void _showPermissionDialog(
     BuildContext context,
     PermissionException denied,