fix(run): turn on tmux mouse mode on every attach, not just at session creation

Sessions created before the mouse-on fix landed (or before the server
restarted to pick it up) never got the set-option call, so their pane
still had no drag/wheel scroll. Moving it into pty-attach's attach()
makes it idempotent per-connection instead of once-at-birth.

server/lib/pty-attach.js

@@ -10,6 +10,8 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
+const tmux = require("./tmux");
+
 const RUN_ID_RE = /^ccam-lane-\d+$/;
 
 /**
@@ -38,6 +40,14 @@ function __setSpawnImpl(fn) {
  */
 function attach(ws, runId, { cols, rows }) {
   validateRunId(runId);
+  // Retroactively covers sessions started before mouse mode was wired into
+  // newSession (or before a server restart picked that change up) — see
+  // tmux.js's enableMouse.
+  try {
+    tmux.enableMouse(runId);
+  } catch {
+    /* session gone or tmux unavailable — attach below will surface that */
+  }
   const pty = spawnImpl("tmux", ["attach-session", "-t", runId], {
     name: "xterm-256color",
     cols: cols || 80,