v0.0.7: Chunked encrypted file transfer

Protocol:
- WireMessage::FileHeader { id, sender_fp, filename, file_size, total_chunks, sha256 }
- WireMessage::FileChunk { id, sender_fp, filename, chunk_index, total_chunks, data }
- 64KB chunks, SHA-256 integrity verification

CLI TUI:
- /file <path> command: reads file, chunks, encrypts each with ratchet, sends
- Progress display: "Sending file.pdf [3/10]..."
- Incoming file reassembly with chunk tracking
- SHA-256 verification on complete
- Saves to data_dir/downloads/
- Max file size: 10MB

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

warzone/crates/warzone-protocol/src/message.rs

@@ -66,4 +66,22 @@ pub enum WireMessage {
         message_id: String,
         receipt_type: ReceiptType,
     },
+    /// File transfer header: announces an incoming chunked file.
+    FileHeader {
+        id: String,
+        sender_fingerprint: String,
+        filename: String,
+        file_size: u64,
+        total_chunks: u32,
+        sha256: String,
+    },
+    /// A single chunk of a file transfer (data is ratchet-encrypted).
+    FileChunk {
+        id: String,
+        sender_fingerprint: String,
+        filename: String,
+        chunk_index: u32,
+        total_chunks: u32,
+        data: Vec<u8>,
+    },
 }