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/src/ui.rs

@@ -147,10 +147,17 @@ fn draw_input(f: &mut Frame, area: ratatui::layout::Rect, app: &App, theme: &The
     ]))
     .block(
         Block::bordered()
-            .border_style(Style::default().fg(theme.border))
+            .border_style(Style::default().fg(if app.pending_offer.is_some() {
+                theme.accent
+            } else {
+                theme.border
+            }))
             .title(Span::styled(
-                " message · enter send · esc quit ",
-                Style::default().fg(theme.dim),
+                match &app.pending_offer {
+                    Some(o) => format!(" ⛧ incoming: {} — /accept or /reject ", o.name),
+                    None => " message · enter send · esc quit ".to_string(),
+                },
+                Style::default().fg(theme.title),
             )),
     );
     f.render_widget(input, area);