fix(install): bring install.sh up to Node 22 and document both Linux scripts (#165)

install.sh still installed Node 20 (dropped in #152), ran the Debian-only
NodeSource script on yum systems, and hard-coded /usr/bin/node. The
README only mentioned install.sh, not setup.sh.

install.sh now:
- installs Node 22 LTS from the right NodeSource repo per distro and
  checks the same 22.12+/24+ floor as setup.sh
- delegates npm install, mirror detection, native-binary checks and the
  build to setup.sh, so the two scripts share one install path
- stops the service and replaces dist/node_modules on re-install (data/
  is kept), copies bin/ (yt-dlp), and uses the real node path in the unit

README explains the difference between the two scripts and when to use
which. setup.sh's "Node.js not found" message no longer says 20+.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/setup.sh

@@ -21,7 +21,7 @@ echo ""
 
 # ---- Check Node.js ----
 if ! command -v node &>/dev/null; then
-    echo "[ERROR] Node.js not found. Please install Node.js 20+ from https://nodejs.org"
+    echo "[ERROR] Node.js not found. Please install Node.js 22.12+ LTS from https://nodejs.org"
     echo "        or https://nodejs.cn/ (China mirror)."
     exit 1
 fi