Web file transfer: send + receive with auto-download

Web client:
- Paperclip file upload button in chat bar
- Chunked upload: 64KB chunks, SHA-256 integrity
- Progress display during send/receive
- Auto-download on complete (browser save dialog)
- Max 10MB per file

WASM:
- decrypt_wire_message now returns file_header and file_chunk
  with type, id, filename, chunk data (hex encoded)

Receive flow:
- FileHeader: registers pending transfer
- FileChunk: stores chunk, shows progress
- All chunks received: assembles, triggers blob download

Send flow (web→web or web→CLI):
- File sent as JSON messages (not bincode, for simplicity)
- Receiver handles both JSON and bincode formats

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/crates/warzone-wasm/src/lib.rs

@@ -448,8 +448,33 @@ pub fn decrypt_wire_message(
                 "receipt_type": rt_str,
             }).to_string())
         }
+        WireMessage::FileHeader {
+            id, sender_fingerprint, filename, file_size, total_chunks, sha256,
+        } => {
+            Ok(serde_json::json!({
+                "type": "file_header",
+                "id": id,
+                "sender": sender_fingerprint,
+                "filename": filename,
+                "file_size": file_size,
+                "total_chunks": total_chunks,
+                "sha256": sha256,
+            }).to_string())
+        }
+        WireMessage::FileChunk {
+            id, sender_fingerprint, filename, chunk_index, total_chunks, data,
+        } => {
+            Ok(serde_json::json!({
+                "type": "file_chunk",
+                "id": id,
+                "sender": sender_fingerprint,
+                "filename": filename,
+                "chunk_index": chunk_index,
+                "total_chunks": total_chunks,
+                "data": hex::encode(&data),
+            }).to_string())
+        }
         _ => {
-            // File transfer messages not yet handled in WASM
             Ok(serde_json::json!({
                 "type": "unsupported",
             }).to_string())