Fix critical issues from PR #681 review

Critical fixes:
- BinaryProtocol: Return nil for unknown versions (prevents buffer underflows)
- Add BinaryProtocol.Offsets struct to centralize magic numbers
- Replace magic offset calculations with named constants

Security/Privacy:
- FileAttachmentView: Use url.lastPathComponent instead of url.path
  (prevents exposing full system paths)

Documentation:
- Fix compression algorithm documentation (zlib, not LZ4)

All tests passing.

bitchat/Protocols/BinaryProtocol.swift

@@ -52,7 +52,7 @@
 /// ## Flag Bits
 /// - Bit 0: Has recipient ID (directed message)
 /// - Bit 1: Has signature (authenticated message)
-/// - Bit 2: Is compressed (LZ4 compression applied)
+/// - Bit 2: Is compressed (zlib compression applied)
 /// - Bits 3-7: Reserved for future use
 ///
 /// ## Size Constraints
@@ -111,11 +111,20 @@ struct BinaryProtocol {
     static let recipientIDSize = 8
     static let signatureSize = 64
 
-    static func headerSize(for version: UInt8) -> Int {
+    // Field offsets within packet header
+    struct Offsets {
+        static let version = 0
+        static let type = 1
+        static let ttl = 2
+        static let timestamp = 3
+        static let flags = 11  // After version(1) + type(1) + ttl(1) + timestamp(8)
+    }
+
+    static func headerSize(for version: UInt8) -> Int? {
         switch version {
         case 1: return v1HeaderSize
         case 2: return v2HeaderSize
-        default: return 0
+        default: return nil
         }
     }
 
@@ -156,7 +165,8 @@ struct BinaryProtocol {
         if version == 1 && payloadDataSize > Int(UInt16.max) { return nil }
         if version == 2 && payloadDataSize > Int(UInt32.max) { return nil }
 
-        let estimatedHeader = headerSize(for: version) + senderIDSize + (packet.recipientID == nil ? 0 : recipientIDSize)
+        guard let headerSize = headerSize(for: version) else { return nil }
+        let estimatedHeader = headerSize + senderIDSize + (packet.recipientID == nil ? 0 : recipientIDSize)
         let estimatedPayload = payloadDataSize
         let estimatedSignature = (packet.signature == nil ? 0 : signatureSize)
         var data = Data()
@@ -274,7 +284,8 @@ struct BinaryProtocol {
 
             guard let version = read8(), version == 1 || version == 2 else { return nil }
             let lengthFieldBytes = lengthFieldSize(for: version)
-            let minimumRequired = headerSize(for: version) + senderIDSize
+            guard let headerSize = headerSize(for: version) else { return nil }
+            let minimumRequired = headerSize + senderIDSize
             guard raw.count >= minimumRequired else { return nil }
 
             guard let type = read8(), let ttl = read8() else { return nil }

bitchat/Services/BLEService.swift

@@ -45,18 +45,16 @@ struct NotificationStreamAssembler {
                 continue
             }
 
-            let headerSize = BinaryProtocol.headerSize(for: version)
-            let framePrefix = headerSize + BinaryProtocol.senderIDSize
-            guard headerSize > 0 else {
+            guard let headerSize = BinaryProtocol.headerSize(for: version) else {
                 dropped.append(buffer.removeFirst())
                 pendingFrameStartedAt = nil
                 pendingFrameExpectedLength = 0
                 continue
             }
+            let framePrefix = headerSize + BinaryProtocol.senderIDSize
             guard buffer.count >= framePrefix else { break }
 
-            let flagsOffset = 1 + 1 + 1 + 8 // version + type + ttl + timestamp
-            let flagsIndex = buffer.startIndex + flagsOffset
+            let flagsIndex = buffer.startIndex + BinaryProtocol.Offsets.flags
             guard flagsIndex < buffer.endIndex else { break }
             let flags = buffer[flagsIndex]
             let hasRecipient = (flags & BinaryProtocol.Flags.hasRecipient) != 0

bitchat/Views/Media/FileAttachmentView.swift

@@ -38,7 +38,7 @@ struct FileAttachmentView: View {
                     .font(.bitchatSystem(size: 14, weight: .medium))
                     .foregroundColor(.primary)
                     .lineLimit(2)
-                Text(url.path)
+                Text(url.lastPathComponent)
                     .font(.bitchatSystem(size: 11, design: .monospaced))
                     .foregroundColor(.secondary)
                     .lineLimit(1)

bitchatTests/NotificationStreamAssemblerTests.swift

@@ -140,9 +140,8 @@ final class NotificationStreamAssemblerTests: XCTestCase {
             return XCTFail("Failed to encode packet frame")
         }
 
-        let flagsOffset = 1 + 1 + 1 + 8
-        XCTAssertLessThan(flagsOffset, frame.count)
-        let flags = frame[frame.startIndex + flagsOffset]
+        XCTAssertLessThan(BinaryProtocol.Offsets.flags, frame.count)
+        let flags = frame[frame.startIndex + BinaryProtocol.Offsets.flags]
         XCTAssertNotEqual(flags & BinaryProtocol.Flags.isCompressed, 0, "Frame should be compressed for large payloads")
 
         let splitIndex = min(4096, frame.count / 2)

bitchatTests/Protocol/BinaryProtocolTests.swift

@@ -90,7 +90,10 @@ final class BinaryProtocolTests: XCTestCase {
         }
         
         // The encoded size should be smaller than uncompressed due to compression
-        let headerSize = BinaryProtocol.headerSize(for: packet.version)
+        guard let headerSize = BinaryProtocol.headerSize(for: packet.version) else {
+            XCTFail("Invalid version")
+            return
+        }
         let uncompressedSize = headerSize + BinaryProtocol.senderIDSize + largePayload.count
         XCTAssertLessThan(encodedData.count, uncompressedSize)