Remove all channel functionality and clean up test suite

- Remove channel UI elements from ContentView
- Remove channel data structures and methods from ChatViewModel
- Remove channel commands (/j, /leave, /channels)
- Remove channel field from BitchatMessage protocol
- Remove channel message types and handling
- Remove NoiseChannelEncryption.swift entirely
- Clean up all channel references across the codebase
- Fix compilation warnings (var to let conversions)
- Remove all outdated test files that used incorrect APIs
- Simplify app to only support public broadcast and 1:1 private messages

bitchat/Noise/NoiseSecurityConsiderations.swift

@@ -61,16 +61,6 @@ struct NoiseSecurityValidator {
                peerID.count <= 64 && 
                peerID.rangeOfCharacter(from: validCharset.inverted) == nil
     }
-    
-    /// Validate channel name format
-    static func validateChannelName(_ channel: String) -> Bool {
-        // Channel should start with # and contain valid characters
-        let validCharset = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "-_"))
-        return channel.hasPrefix("#") && 
-               channel.count > 1 && 
-               channel.count <= 32 &&
-               channel.dropFirst().rangeOfCharacter(from: validCharset.inverted) == nil
-    }
 }
 
 // MARK: - Enhanced Noise Session with Security
@@ -232,54 +222,6 @@ enum NoiseSecurityError: Error {
     case sessionExhausted
     case messageTooLarge
     case invalidPeerID
-    case invalidChannelName
     case rateLimitExceeded
     case handshakeTimeout
 }
-
-// MARK: - Security Audit Checklist
-
-/*
- SECURITY AUDIT CHECKLIST:
- 
- 1. KEY MANAGEMENT
-    ✓ Static keys stored in Keychain (most secure iOS storage)
-    ✓ Keys cleared on panic mode
-    ✓ Ephemeral keys generated per session
-    ✓ No key reuse across sessions
- 
- 2. PROTOCOL SECURITY
-    ✓ Using Noise XX pattern for mutual authentication
-    ✓ Forward secrecy via ephemeral keys
-    ✓ Replay protection via nonce counter
-    ✓ AEAD encryption (ChaCha20-Poly1305)
-    ✓ SHA-256 for hashing
- 
- 3. IMPLEMENTATION SECURITY
-    ✓ Message size limits to prevent DoS
-    ✓ Session timeout to limit exposure
-    ✓ Message count limits to prevent nonce reuse
-    ✓ Rate limiting for handshakes and messages
-    ✓ Input validation for all user data
-    ✓ Thread-safe operations
- 
- 4. NETWORK SECURITY
-    ✓ Messages padded to standard sizes for traffic analysis resistance
-    ✓ Cover traffic for anonymity
-    ✓ No metadata leakage in protocol
-    ✓ Fingerprint verification for identity
- 
- 5. EDGE CASES HANDLED
-    ✓ Incomplete handshakes timeout
-    ✓ Duplicate handshake messages ignored
-    ✓ Session renegotiation when needed
-    ✓ Graceful handling of decryption failures
-    ✓ Memory limits on message sizes
- 
- 6. FUTURE IMPROVEMENTS
-    - Implement post-quantum key exchange (when available)
-    - Add perfect forward secrecy for channel keys
-    - Implement key rotation for long-lived channels
-    - Add security event logging
-    - Implement secure key backup/restore
- */