Fix silent error handling in message encryption

Previously, if encryption failed for any recipient, the error was only
logged to console and the code continued, resulting in a message being
sent that some recipients couldn't decrypt. This creates a security and
reliability issue.

Changes:
- Collect all failed encryption attempts instead of silently logging
- Throw an error if any recipient encryption fails
- Provide detailed error message listing number of failures
- Implement all-or-nothing approach to prevent partial message delivery

This ensures senders are notified immediately if message encryption
fails for any recipient, preventing silent security failures.

src/frontend/utils/crypto.js

@@ -175,6 +175,7 @@ export async function encryptMessage(message, recipientPublicKeys) {
 
   // Encrypt AES key for each recipient with their RSA public key
   const encryptedKeys = {};
+  const failedRecipients = [];
 
   for (const recipientKeyB64 of recipientPublicKeys) {
     try {
@@ -201,10 +202,22 @@ export async function encryptMessage(message, recipientPublicKeys) {
 
       encryptedKeys[recipientKeyB64] = arrayBufferToBase64(encryptedKey);
     } catch (error) {
-      console.error('Failed to encrypt for recipient:', error);
+      failedRecipients.push({
+        publicKey: recipientKeyB64.substring(0, 20) + '...', // Truncate for error message
+        error: error.message
+      });
     }
   }
 
+  // Throw error if any encryptions failed to prevent partial message delivery
+  if (failedRecipients.length > 0) {
+    throw new Error(
+      `Failed to encrypt message for ${failedRecipients.length} recipient(s): ${
+        failedRecipients.map(r => r.error).join(', ')
+      }`
+    );
+  }
+
   return {
     ciphertext: arrayBufferToBase64(encryptedMessage),
     iv: arrayBufferToBase64(iv),