feat(installer): add -version flag to pin 3x-ui release

Accepts 3.4.2 or v3.4.2; validates the tag exists on GitHub before
download, falls back to latest when omitted.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README_EN.md

@@ -88,6 +88,7 @@ bash x-ui-latest.sh -uninstall y
 | `-subdomain <domain>` | Panel and subscription domain |
 | `-reality_domain <domain>` | REALITY destination domain |
 | `-auto_domain y` | Auto-detect domain (no manual input) |
+| `-version <version>` | Install specific 3x-ui version (e.g. `3.4.2`), default — latest |
 | `-uninstall y` | Full uninstall |
 
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