fix(crawler): translate socks5h:// → socks5:// for Chromium --proxy-server

Chromium doesn't know the socks5h scheme (curl/reqwest convention)
and bails navigations with ERR_NO_SUPPORTED_PROXIES. It does, however,
send destination hostnames over SOCKS5 by default, so stripping the
`h` is a pure scheme rename — remote-DNS behaviour is preserved.

reqwest keeps the user's original CRAWLER_PROXY string (`socks5h://...`
remains valid and meaningful for it).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/bin/crawler.rs

@@ -127,7 +127,8 @@ async fn main() -> anyhow::Result<()> {
 
     let mut options = LaunchOptions::from_env();
     if let Some(proxy) = &proxy_url {
-        options.extra_args.push(format!("--proxy-server={proxy}"));
+        let chromium_proxy = mangalord::crawler::url_utils::chromium_proxy_arg(proxy);
+        options.extra_args.push(format!("--proxy-server={chromium_proxy}"));
     }
     let keep_open = match (keep_browser_open, options.mode) {
         (true, BrowserMode::Headed) => true,