fix(security): prevent notifications from blocked users [BCH-01-012]

Block notifications from bypassing the user blocking feature on iOS.

Vulnerabilities fixed:
1. Nostr public messages: checkForMentions() was called regardless of
   blocking status, allowing blocked users to trigger mention notifications
2. Noise-encrypted DMs: didReceiveNoisePayload() didn't check blocking
   before calling handlePrivateMessage(), allowing DM notifications

Changes:
- ChatViewModel+Nostr.swift: Add blocking check before checkForMentions()
  and sendHapticFeedback() in subscribeNostrEvent
- ChatViewModel.swift: Add isPeerBlocked() check in didReceiveNoisePayload
  before processing private messages
- Add NotificationBlockingTests.swift with 5 tests for blocking behavior

Security: Blocked users can no longer trigger notifications through any
message path (Nostr public, Nostr DM, or BLE Noise DM).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bitchat/ViewModels/ChatViewModel.swift

@@ -3052,20 +3052,27 @@ final class ChatViewModel: ObservableObject, BitchatDelegate, CommandContextProv
             switch type {
             case .privateMessage:
                 guard let pm = PrivateMessagePacket.decode(from: payload) else { return }
+
+                // BCH-01-012: Check blocking before processing private message to prevent notification bypass
+                if isPeerBlocked(peerID) {
+                    SecureLogger.debug("🚫 Ignoring Noise payload from blocked peer: \(peerID)", category: .security)
+                    return
+                }
+
                 let senderName = unifiedPeerService.getPeer(by: peerID)?.nickname ?? "Unknown"
-            let pmMentions = parseMentions(from: pm.content)
-            let msg = BitchatMessage(
-                id: pm.messageID,
-                sender: senderName,
-                content: pm.content,
-                timestamp: timestamp,
-                isRelay: false,
-                originalSender: nil,
-                isPrivate: true,
-                recipientNickname: nickname,
-                senderPeerID: peerID,
-                mentions: pmMentions.isEmpty ? nil : pmMentions
-            )
+                let pmMentions = parseMentions(from: pm.content)
+                let msg = BitchatMessage(
+                    id: pm.messageID,
+                    sender: senderName,
+                    content: pm.content,
+                    timestamp: timestamp,
+                    isRelay: false,
+                    originalSender: nil,
+                    isPrivate: true,
+                    recipientNickname: nickname,
+                    senderPeerID: peerID,
+                    mentions: pmMentions.isEmpty ? nil : pmMentions
+                )
                 handlePrivateMessage(msg)
                 // Send delivery ACK back over BLE
                 meshService.sendDeliveryAck(for: pm.messageID, to: peerID)

bitchat/ViewModels/Extensions/ChatViewModel+Nostr.swift

@@ -130,12 +130,21 @@ extension ChatViewModel {
             mentions: mentions.isEmpty ? nil : mentions
         )
         Task { @MainActor in
+            // BCH-01-012: Check blocking before any notifications
+            // handlePublicMessage has its own blocking check but returns silently,
+            // so we must also guard checkForMentions to prevent notification bypass
+            let isBlocked = identityManager.isNostrBlocked(pubkeyHexLowercased: event.pubkey.lowercased())
+
             handlePublicMessage(msg)
-            checkForMentions(msg)
-            sendHapticFeedback(for: msg)
+
+            // Only check mentions and send haptic if sender is not blocked
+            if !isBlocked {
+                checkForMentions(msg)
+                sendHapticFeedback(for: msg)
+            }
         }
     }
-    
+
     func subscribeGiftWrap(_ giftWrap: NostrEvent, id: NostrIdentity) {
         guard !deduplicationService.hasProcessedNostrEvent(giftWrap.id) else { return }
         deduplicationService.recordNostrEvent(giftWrap.id)