feat(install): support version-pinned install from release assets

Add PINNED_VERSION mechanism so scripts downloaded from a specific
release (e.g. 2.1.0) install that exact version instead of latest.
CI injects the version via sed during release build. Users can still
override with VERSION= env var. Update release notes and README to
show both pinned and latest install options.

panel_install.sh

@@ -10,6 +10,9 @@ export LC_ALL=C
 # GitHub repo used for release downloads
 REPO="Sagit-chu/flux-panel"
 
+# 固定版本号（Release 构建时自动填充，留空则获取最新版）
+PINNED_VERSION=""
+
 COUNTRY=$(curl -s https://ipinfo.io/country)
 
 maybe_proxy_url() {
@@ -69,6 +72,10 @@ resolve_version() {
     echo "$FLUX_VERSION"
     return 0
   fi
+  if [[ -n "${PINNED_VERSION:-}" ]]; then
+    echo "$PINNED_VERSION"
+    return 0
+  fi
 
   if resolve_latest_release_tag; then
     return 0