feat(hh): P5 — file & directory uploads ⛧

Wire-compatible with the Python client's _ft protocol (offer/accept/reject/
chunk/done, 64KB chunks, SHA-256 verified), over the encrypted channel:
- ft.rs: read_payload (file or tar'd directory), save/extract with a zip-slip
  guard (relative-only, no .. / absolute; unpack_in double-checks), SHA-256.
- /send <file> and /sendd <dir>; receiver sees an offer banner, /accept or
  /reject; chunks stream in the background and the result is verified + saved
  to ./downloads (directories extracted in place).
- Refactor: all outgoing ws frames now funnel through an mpsc channel drained
  (batched) by the run loop, so the background chunk-sender and the PTY relay
  transmit without owning the socket.
- ui.rs: pending-offer banner on the input bar.

Tests: 7 cargo tests (file + dir tar round-trip, traversal guard, + crypto/sbx).
Verified live: two TUIs, file and directory transfer, SHA-256 verified, saved.

Note: dropping accepted files into the active sandbox VM dir is a future add-on.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hh/Cargo.toml

@@ -30,6 +30,9 @@ url = "2"
 portable-pty = "0.8"
 vt100 = "0.15"
 
+# file/dir transfer (P5)
+tar = "0.4"
+
 # async + tui
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "io-util", "sync", "time"] }
 tokio-tungstenite = { version = "0.24", features = ["rustls-tls-webpki-roots"] }