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/__tests__/tmux.test.js

@@ -47,6 +47,7 @@ describe("tmux wrapper", () => {
       "--model",
       "opus",
     ]);
+    assert.deepEqual(calls[1], ["set-option", "-t", "ccam-lane-1", "mouse", "on"]);
   });
 
   it("killSession never throws when the session is already gone", () => {