feat(storage): support configurable install root

Add a LuCI install-root input, persist the selected path in UCI,
and route install, status, backup, uninstall, and runtime scripts
through the configured storage root for new installs.

Reference: custom install root flow

root/etc/config/openclaw

@@ -1,5 +1,6 @@
 config openclaw 'main'
 	option enabled '0'
+	option install_root '/opt'
 	option port '18789'
 	option bind 'lan'
 	option token ''