Managed Node runtime cannot be replaced when the bundled binary will not execute

**Describe the bug**

Buzz Desktop cannot use a packager-supplied Node.js runtime for npm-backed ACP adapters. On NixOS, Buzz downloads the official Node.js tarball successfully, but that generic ELF cannot run because it expects `/lib64/ld-linux-x86-64.so.2` and shared libraries outside the Nix store. The UI then reports a readiness failure and suggests checking the network or app-data permissions, even though download, checksum verification, extraction, and permissions all succeeded.

**Steps to reproduce**

1. Run Buzz Desktop on NixOS without `programs.nix-ld`.
2. Remove any existing private runtime under `~/.local/share/Buzz/runtimes/node`.
3. Open Settings, then Agents.
4. Install an npm-backed ACP adapter such as Codex ACP.
5. Buzz downloads `node-v24.18.0-linux-x64.tar.gz`, then reports that the managed Node.js runtime did not pass readiness after install.

**Expected behavior**

Packagers that already provide Node.js 24 should have a declarative way to point Buzz at that trusted runtime. An explicitly configured but invalid runtime should fail with a specific error and should not fall back to the downloaded tarball or an arbitrary executable from `PATH`.

**Version and platform**

- Buzz version: 0.5.20
- OS: NixOS 26.11, x86_64
- Packaged Node.js: 24.19.0

**Logs / additional context**

The downloaded binary exits 127 through NixOS stub-ld. Its ELF interpreter is `/lib64/ld-linux-x86-64.so.2`, and `ldd` also reports a missing `libstdc++.so.6`. Buzz currently builds only its private versioned runtime path and requires `node --version` to equal `v24.18.0`, so substituting nixpkgs `nodejs_24` is not supported either.

I propose an opt-in `BUZZ_NODE_BIN_DIR` setting. It would accept an absolute directory containing executable `node` and `npm`, require a well-formed Node 24 version, and bypass the download only when explicitly set. Buzz's own SHA-pinned runtime would keep its exact `v24.18.0` check. With the variable unset, behavior on macOS, Windows, and supported Linux systems would not change.

This is related to the portability problem in #2604, but it does not fix that AppImage issue. Packaging work in #3734 and #6924 could consume the new setting.

Read the managed Node runtime readiness path used by Settings > Agents; trace how the private versioned runtime is selected and how node --version is checked. Verify the requested BUZZ_NODE_BIN_DIR behavior on NixOS, including invalid-runtime errors, no fallback, and unchanged behavior when unset.

Written by the indexing model from the issue text.