tui: render keyboard icon with emoji variation selector (⌨️) (#2728)

Use emoji variation selector (VS16) for the keyboard icon so it
consistently renders as emoji (⌨️) rather than text (⌨) across
terminals.

Touches TUI command rendering for unknown parsed commands. No behavior
change beyond display.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__interrupt_exec_marks_failed.snap

@@ -3,4 +3,4 @@ source: tui/src/chatwidget/tests.rs
 expression: exec_blob
 ---
 >_
-  ✗ ⌨  sleep 1
+  ✗ ⌨️  sleep 1