Pre-install meson-python+ninja before --no-build-isolation step

hmr2/skel have transitive deps that use meson-python as build backend.
With --no-build-isolation pip cannot auto-fetch build backends into an
isolated env, so mesonpy must already be present in the main env.

app.py

@@ -68,6 +68,13 @@ def _install_runtime_packages():
     if _RUNTIME_PKG_MARKER.exists():
         return
     print("[startup] Installing runtime packages (first run, ~10-15 min)...")
+    # With --no-build-isolation pip uses the main env for all build backends.
+    # Pre-install meson-python + ninja so meson-based transitive deps (e.g. from
+    # hmr2/skel's dep tree) can be built without a separate isolated env.
+    subprocess.run(
+        [sys.executable, "-m", "pip", "install", "--quiet",
+         "meson-python", "ninja"], check=True,
+    )
     subprocess.run(
         [sys.executable, "-m", "pip", "install", "--quiet", "--no-build-isolation"]
         + _NO_ISOLATION_PACKAGES, check=True,