v0.0.20: file transfer in groups

/file <path> now works in group mode (#group):
- Sends file header + chunks to each group member
- Same fan-out approach as group text messages
- Each member receives and reassembles independently
- Progress shown: "Sending 'file.pdf' to group #ops..."

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

warzone/Cargo.toml

@@ -9,7 +9,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.0.19"
+version = "0.0.20"
 edition = "2021"
 license = "MIT"
 rust-version = "1.75"