fix(run): enable tmux mouse mode so the run terminal's scrollbar actually drags

Session panes had a scrollbar track but no way to drag it — wheel/drag
events passed straight to the running program instead of entering tmux's
copy-mode scrollback. xterm.js already forwards mouse-tracking escapes
once tmux advertises them, so turning mouse mode on per-session is enough.

server/lib/tmux.js

@@ -40,6 +40,11 @@ function hasSession(name) {
  */
 function newSession({ name, cwd, argv }) {
   execImpl(["new-session", "-d", "-s", name, "-c", cwd, "--", ...argv]);
+  // Without this the pane has no scrollbar/drag scroll at all — wheel events
+  // just pass through to the running program instead of entering tmux's own
+  // copy-mode scrollback. xterm.js forwards tmux's mouse-tracking escapes
+  // automatically once mouse mode is on, so no client-side change is needed.
+  execImpl(["set-option", "-t", name, "mouse", "on"]);
 }
 
 /**