Implement hashtag rooms feature

- Add room field to BitchatMessage structure
- Update BinaryProtocol to encode/decode room information
- Create room management in ChatViewModel (join/leave/switch rooms)
- Add BluetoothMeshService support for room messages
- Implement rooms UI in sidebar with unread counts
- Create clickable hashtags in messages to join rooms
- Add room persistence across app restarts
- Filter message display by current room selection

bitchat/Protocols/BinaryProtocol.swift

@@ -193,6 +193,7 @@ extension BitchatMessage {
         // - Recipient nickname length + data
         // - Sender peer ID length + data
         // - Mentions array
+        // - Room hashtag
         
         var flags: UInt8 = 0
         if isRelay { flags |= 0x01 }
@@ -201,6 +202,7 @@ extension BitchatMessage {
         if recipientNickname != nil { flags |= 0x08 }
         if senderPeerID != nil { flags |= 0x10 }
         if mentions != nil && !mentions!.isEmpty { flags |= 0x20 }
+        if room != nil { flags |= 0x40 }
         
         data.append(flags)
         
@@ -267,6 +269,12 @@ extension BitchatMessage {
             }
         }
         
+        // Room hashtag
+        if let room = room, let roomData = room.data(using: .utf8) {
+            data.append(UInt8(min(roomData.count, 255)))
+            data.append(roomData.prefix(255))
+        }
+        
         return data
     }
     
@@ -292,6 +300,7 @@ extension BitchatMessage {
         let hasRecipientNickname = (flags & 0x08) != 0
         let hasSenderPeerID = (flags & 0x10) != 0
         let hasMentions = (flags & 0x20) != 0
+        let hasRoom = (flags & 0x40) != 0
         
         // Timestamp
         guard offset + 8 <= dataCopy.count else { 
@@ -389,6 +398,16 @@ extension BitchatMessage {
             }
         }
         
+        // Room
+        var room: String? = nil
+        if hasRoom && offset < dataCopy.count {
+            let length = Int(dataCopy[offset]); offset += 1
+            if offset + length <= dataCopy.count {
+                room = String(data: dataCopy[offset..<offset+length], encoding: .utf8)
+                offset += length
+            }
+        }
+        
         let message = BitchatMessage(
             sender: sender,
             content: content,
@@ -398,7 +417,8 @@ extension BitchatMessage {
             isPrivate: isPrivate,
             recipientNickname: recipientNickname,
             senderPeerID: senderPeerID,
-            mentions: mentions
+            mentions: mentions,
+            room: room
         )
         return message
     }

bitchat/Protocols/BitchatProtocol.swift

@@ -133,8 +133,9 @@ struct BitchatMessage: Codable, Equatable {
     let recipientNickname: String?
     let senderPeerID: String?
     let mentions: [String]?  // Array of mentioned nicknames
+    let room: String?  // Room hashtag (e.g., "#general")
     
-    init(sender: String, content: String, timestamp: Date, isRelay: Bool, originalSender: String? = nil, isPrivate: Bool = false, recipientNickname: String? = nil, senderPeerID: String? = nil, mentions: [String]? = nil) {
+    init(sender: String, content: String, timestamp: Date, isRelay: Bool, originalSender: String? = nil, isPrivate: Bool = false, recipientNickname: String? = nil, senderPeerID: String? = nil, mentions: [String]? = nil, room: String? = nil) {
         self.id = UUID().uuidString
         self.sender = sender
         self.content = content
@@ -145,6 +146,7 @@ struct BitchatMessage: Codable, Equatable {
         self.recipientNickname = recipientNickname
         self.senderPeerID = senderPeerID
         self.mentions = mentions
+        self.room = room
     }
 }