Implement KeeShare support for group synchronization

Add receiving-end support for KeeShare to enable secure sharing of
password groups between databases. This feature allows automatic
synchronization of shared groups when the database is opened.

Key features:
- Parse KeeShare reference metadata from group CustomData
- Handle both raw .kdbx files and .share container format (zip)
- RSA signature verification using SHA-256 for secure imports
- Automatic merge using KeePassLib's native Synchronize method
- Integration hook in Database.LoadData for seamless operation

The implementation follows KeePassXC's KeeShare specification and
provides the import functionality requested by users for sharing
passwords across devices and users.

Fixes #1161

Author: natinew77-creator

src/Kp2aBusinessLogic/KeeShare/IKeeShareUserInteraction.cs

@@ -0,0 +1,111 @@
+using System;
+using System.Threading.Tasks;
+
+namespace keepass2android.KeeShare
+{
+    /// <summary>
+    /// Result of a user's trust decision for an untrusted signer
+    /// </summary>
+    public enum TrustDecision
+    {
+        /// <summary>Trust this signer permanently (add to trusted keys)</summary>
+        TrustPermanently,
+        /// <summary>Trust this signer for this session only</summary>
+        TrustOnce, 
+        /// <summary>Reject this signer (do not import)</summary>
+        Reject,
+        /// <summary>User cancelled the dialog</summary>
+        Cancel
+    }
+
+    /// <summary>
+    /// Information about an untrusted signer presented to the user
+    /// </summary>
+    public class UntrustedSignerInfo
+    {
+        /// <summary>Name of the signer from the signature file</summary>
+        public string SignerName { get; set; }
+        
+        /// <summary>SHA-256 fingerprint of the public key (hex, lowercase)</summary>
+        public string KeyFingerprint { get; set; }
+        
+        /// <summary>Path to the share file</summary>
+        public string SharePath { get; set; }
+        
+        /// <summary>
+        /// Get a formatted fingerprint for display (e.g., "AB:CD:EF:12:...")
+        /// </summary>
+        public string FormattedFingerprint
+        {
+            get
+            {
+                if (string.IsNullOrEmpty(KeyFingerprint) || KeyFingerprint.Length < 2)
+                    return KeyFingerprint;
+                    
+                // Format as colon-separated pairs for readability
+                var result = new System.Text.StringBuilder();
+                for (int i = 0; i < KeyFingerprint.Length; i += 2)
+                {
+                    if (i > 0) result.Append(':');
+                    result.Append(KeyFingerprint.Substring(i, Math.Min(2, KeyFingerprint.Length - i)).ToUpperInvariant());
+                }
+                return result.ToString();
+            }
+        }
+    }
+
+    /// <summary>
+    /// Interface for handling user prompts during KeeShare import.
+    /// Implement this in the Android UI layer to show dialogs to the user.
+    /// </summary>
+    public interface IKeeShareUserInteraction
+    {
+        /// <summary>
+        /// Prompt the user to trust an unknown signer.
+        /// Called when a share file is signed by a key not in the trusted store.
+        /// </summary>
+        /// <param name="signerInfo">Information about the untrusted signer</param>
+        /// <returns>User's trust decision</returns>
+        Task<TrustDecision> PromptTrustDecisionAsync(UntrustedSignerInfo signerInfo);
+        
+        /// <summary>
+        /// Notify the user that imports were completed.
+        /// Called after CheckAndImport finishes processing all shares.
+        /// </summary>
+        /// <param name="results">List of import results</param>
+        void NotifyImportResults(System.Collections.Generic.List<KeeShareImportResult> results);
+        
+        /// <summary>
+        /// Check if auto-import is enabled in user preferences.
+        /// If false, shares will not be imported automatically on database load.
+        /// </summary>
+        bool IsAutoImportEnabled { get; }
+    }
+    
+    /// <summary>
+    /// Default implementation that rejects all untrusted signers (no UI).
+    /// Use this as a fallback when no UI handler is registered.
+    /// </summary>
+    public class DefaultKeeShareUserInteraction : IKeeShareUserInteraction
+    {
+        public Task<TrustDecision> PromptTrustDecisionAsync(UntrustedSignerInfo signerInfo)
+        {
+            // No UI available - reject by default for security
+            Kp2aLog.Log($"KeeShare: No UI handler registered. Rejecting untrusted signer '{signerInfo?.SignerName}'");
+            return Task.FromResult(TrustDecision.Reject);
+        }
+        
+        public void NotifyImportResults(System.Collections.Generic.List<KeeShareImportResult> results)
+        {
+            // No UI - just log
+            if (results == null) return;
+            foreach (var result in results)
+            {
+                if (result.IsSuccess)
+                    Kp2aLog.Log($"KeeShare: Imported {result.EntriesImported} entries from {result.SharePath}");
+            }
+        }
+        
+        public bool IsAutoImportEnabled => true; // Default to enabled for backward compatibility
+    }
+}