fix(luci): ensure Tachyon menu item reliably appears in LuCI

- Remove 'uci: { tachyon: true }' dependency from menu.d/luci-app-tachyon.json so LuCI does not hide the menu node if UCI config is uncommitted/reloading
- Add portable timeout normalization in install.sh ucode to handle Busybox timeout and systems without timeout applet
- Ensure luci_postinst is called directly in IPK postinst (build.sh) and after install in install.sh
- Restart uhttpd alongside rpcd and purge LuCI caches in post-install and uninstaller
- Fix uninstall.sh to remove packages individually with lock retry and omit non-existent luci-i18n-tachyon-en
- In install_ui_packages, verify that menu.d/luci-app-tachyon.json actually exists on disk before proceeding

build.sh

@@ -466,6 +466,7 @@ EOF
 [ -s ${IPKG_INSTROOT}/lib/functions.sh ] || exit 0
 . ${IPKG_INSTROOT}/lib/functions.sh
 default_postinst $0 $@
+[ -n "${IPKG_INSTROOT}" ] || /usr/bin/tachyon luci_postinst >/dev/null 2>&1 || true
 EOF
 
   cat > "$control_dir/postinst-pkg" <<'EOF'