Optimize UI performance with message caching and List view

- Convert BitchatMessage from struct to class for efficient caching
- Cache formatted AttributedStrings to avoid expensive regex on every render
- Replace ScrollView+LazyVStack with native List for better cell reuse
- Implement message windowing (show last 100 messages for performance)
- Add LazyLinkPreviewView that defers loading for 0.5s to improve scroll performance

These optimizations significantly improve scroll performance, especially with large message lists.

bitchat/Protocols/BitchatProtocol.swift

@@ -929,7 +929,7 @@ enum DeliveryStatus: Codable, Equatable {
     }
 }
 
-struct BitchatMessage: Codable, Equatable {
+class BitchatMessage: Codable {
     let id: String
     let sender: String
     let content: String
@@ -942,6 +942,23 @@ struct BitchatMessage: Codable, Equatable {
     let mentions: [String]?  // Array of mentioned nicknames
     var deliveryStatus: DeliveryStatus? // Delivery tracking
     
+    // Cached formatted text (not included in Codable)
+    private var _cachedFormattedText: [String: AttributedString] = [:]
+    
+    func getCachedFormattedText(isDark: Bool) -> AttributedString? {
+        return _cachedFormattedText["\(isDark)"]
+    }
+    
+    func setCachedFormattedText(_ text: AttributedString, isDark: Bool) {
+        _cachedFormattedText["\(isDark)"] = text
+    }
+    
+    // Codable implementation
+    enum CodingKeys: String, CodingKey {
+        case id, sender, content, timestamp, isRelay, originalSender
+        case isPrivate, recipientNickname, senderPeerID, mentions, deliveryStatus
+    }
+    
     init(id: String? = nil, sender: String, content: String, timestamp: Date, isRelay: Bool, originalSender: String? = nil, isPrivate: Bool = false, recipientNickname: String? = nil, senderPeerID: String? = nil, mentions: [String]? = nil, deliveryStatus: DeliveryStatus? = nil) {
         self.id = id ?? UUID().uuidString
         self.sender = sender
@@ -957,6 +974,23 @@ struct BitchatMessage: Codable, Equatable {
     }
 }
 
+// Equatable conformance for BitchatMessage
+extension BitchatMessage: Equatable {
+    static func == (lhs: BitchatMessage, rhs: BitchatMessage) -> Bool {
+        return lhs.id == rhs.id &&
+               lhs.sender == rhs.sender &&
+               lhs.content == rhs.content &&
+               lhs.timestamp == rhs.timestamp &&
+               lhs.isRelay == rhs.isRelay &&
+               lhs.originalSender == rhs.originalSender &&
+               lhs.isPrivate == rhs.isPrivate &&
+               lhs.recipientNickname == rhs.recipientNickname &&
+               lhs.senderPeerID == rhs.senderPeerID &&
+               lhs.mentions == rhs.mentions &&
+               lhs.deliveryStatus == rhs.deliveryStatus
+    }
+}
+
 protocol BitchatDelegate: AnyObject {
     func didReceiveMessage(_ message: BitchatMessage)
     func didConnectToPeer(_ peerID: String)