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/__tests__/pty-attach.test.js

@@ -5,10 +5,11 @@
  * or PTY spawned).
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
-const { describe, it, beforeEach } = require("node:test");
+const { describe, it, beforeEach, afterEach } = require("node:test");
 const assert = require("node:assert/strict");
 const { EventEmitter } = require("node:events");
 const ptyAttach = require("../lib/pty-attach");
+const tmux = require("../lib/tmux");
 
 function makeFakePty() {
   const emitter = new EventEmitter();
@@ -41,6 +42,10 @@ describe("pty-attach", () => {
   beforeEach(() => {
     fakePty = makeFakePty();
     ptyAttach.__setSpawnImpl(() => fakePty);
+    tmux.__setExecImpl(() => "");
+  });
+  afterEach(() => {
+    tmux.__reset();
   });
 
   it("rejects a runId that doesn't match ccam-lane-<digits>", () => {
@@ -52,6 +57,25 @@ describe("pty-attach", () => {
     assert.doesNotThrow(() => ptyAttach.validateRunId("ccam-lane-42"));
   });
 
+  it("turns tmux mouse mode on for the session it attaches to", () => {
+    const calls = [];
+    tmux.__setExecImpl((args) => {
+      calls.push(args);
+      return "";
+    });
+    ptyAttach.attach(makeFakeWs(), "ccam-lane-1", { cols: 80, rows: 24 });
+    assert.deepEqual(calls[0], ["set-option", "-t", "ccam-lane-1", "mouse", "on"]);
+  });
+
+  it("still attaches even if the mouse-mode set-option call fails", () => {
+    tmux.__setExecImpl(() => {
+      throw new Error("no such session: ccam-lane-1");
+    });
+    assert.doesNotThrow(() =>
+      ptyAttach.attach(makeFakeWs(), "ccam-lane-1", { cols: 80, rows: 24 })
+    );
+  });
+
   it("wires PTY data to binary WS frames and WS binary frames to PTY writes", () => {
     const ws = makeFakeWs();
     ptyAttach.attach(ws, "ccam-lane-1", { cols: 80, rows: 24 });