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.

bitchatTests/Protocol/BinaryProtocolTests.swift

@@ -68,7 +68,10 @@ struct BinaryProtocolTests {
         let encodedData = try #require(BinaryProtocol.encode(packet), "Failed to encode packet with large payload")
         
         // 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
         #expect(encodedData.count < uncompressedSize, "Compressed packet should be smaller than uncompressed form")