fix(desktop): self-update rebuilds and relaunches cleanly on macOS

The macOS DMG / in-app update could leave Hermes unable to relaunch: the
staged updater rebuilt the desktop without managed Node on PATH ("npm not
found"), never installed the rebuilt bundle over the running app, and could
race itself on `git stash`. Child install scripts also inherited a deleted
cwd from the .app bundle replaced during self-update.

- update.rs: prepend $HERMES_HOME/node/bin + venv bin to the rebuild PATH;
  read --branch / --target-app from args; add a macOS "install" stage that
  dittos the rebuilt bundle over the target app, clears quarantine, and
  relaunches via `open` (rolling back on a failed swap); guard start_update
  with an AtomicBool so concurrent startUpdate() calls can't race git stash.
- main.cjs: pass --branch <configured> and --target-app <running bundle> to
  the staged updater, and spawn it with HERMES_HOME + managed Node/venv on
  PATH and cwd=HERMES_HOME.
- bootstrap.rs: launch the desktop via `open <App>.app` on macOS instead of
  exec'ing Contents/MacOS/Hermes, avoiding cwd/quarantine issues post-rebuild.
- powershell.rs: pin child install scripts to a stable cwd so they don't emit
  getcwd errors when the launching .app is replaced mid-install.
- failure.tsx: in update mode show "Update didn't finish" / "Retry update"
  and retry via startUpdate() instead of re-running the installer bootstrap.

apps/bootstrap-installer/src-tauri/src/powershell.rs

@@ -45,6 +45,14 @@ pub async fn run_script(
 ) -> Result<ScriptResult> {
     let mut cmd = build_command(script_path, args);
 
+    // The installer can be launched from a .app bundle that is later replaced
+    // during self-update. Pin child scripts to a stable directory so bash/zsh
+    // never starts from a deleted cwd and emits getcwd/job-working-directory
+    // errors at the end of an otherwise successful install.
+    if let Some(cwd) = stable_script_cwd(script_path, hermes_home_override) {
+        cmd.current_dir(cwd);
+    }
+
     if let Some(home) = hermes_home_override {
         cmd.env("HERMES_HOME", home);
     }
@@ -146,6 +154,16 @@ pub async fn run_script(
     })
 }
 
+fn stable_script_cwd<'a>(script_path: &'a Path, hermes_home_override: Option<&'a str>) -> Option<&'a Path> {
+    if let Some(home) = hermes_home_override {
+        let path = Path::new(home);
+        if path.is_dir() {
+            return Some(path);
+        }
+    }
+    script_path.parent().filter(|p| p.is_dir())
+}
+
 async fn recv_cancel(rx: &mut Option<CancelRx>) {
     match rx {
         Some(r) => {
@@ -264,4 +282,11 @@ info line
         assert!(parse_stage_result("just banner\n").is_none());
         assert!(parse_manifest("just banner\n").is_none());
     }
+
+    #[test]
+    fn stable_script_cwd_prefers_existing_hermes_home() {
+        let script = Path::new("/tmp/install.sh");
+        let cwd = stable_script_cwd(script, Some("/"));
+        assert_eq!(cwd, Some(Path::new("/")));
+    }
 }