fix: v2.5.6 — Windows panes without sh, UI build without node on PATH

- Windows panes: a WinGet tmux runs native programs and there is no `sh`, so
  every pane's `sh -c` wrapper died at once ("Command failed: tmux set-option
  … mouse on"). paneArgv runs the CLI itself on win32, and a Terminal gets
  tmux's own default shell (newSession drops the `--` for an empty argv).
- Plugin UI build: client deps install with --ignore-scripts and tsc/vite run
  by the running node instead of `npm run build`. On Windows esbuild's
  postinstall went through cmd.exe with no `node` on the server's PATH, both
  installs failed, and Update now kept the previous UI. A test keeps the two
  build steps equal to client/package.json's build script.
- Update all: Atomics.wait instead of a `sleep` binary Windows lacks.

server/__tests__/pty-run.test.js

@@ -499,6 +499,15 @@ describe("pty-run", () => {
     return argv.slice(4);
   }
 
+  it("runs a Windows pane's command itself — no sh there — and a Terminal as tmux's default shell", () => {
+    const argv = ["C:\\Users\\u\\.local\\bin\\claude.exe", "--model", "opus"];
+    assert.deepEqual(pty.paneArgv(argv, "win32"), argv);
+    assert.deepEqual(pty.paneArgv(null, "win32"), []);
+    assert.equal(pty.paneArgv(argv, "linux")[0], "sh");
+    assert.deepEqual(pty.paneArgv(argv, "linux").slice(-3), argv);
+    assert.match(pty.paneArgv(null, "linux")[2], /exec "\$SHELL" -l$/);
+  });
+
   it("spawnRun starts the CLI only once the pane has its real size, argv untouched", () => {
     // A claude drawn at tmux's birth size duplicated its banner on the first
     // resize; the argv rides as positional parameters, never script text.