fix: apply protocol-matched User-Agent for subscription fetches (#5)

Subscription panels (v2board/xboard/sspanel) route responses by UA.
The previous client sent no UA (default reqwest/0.12.x) so these panels
rejected the request, surfacing as "address unreachable" even though
the host was reachable (sing-box SFM with a proper UA worked fine).

- loader: apply user_agent to reqwest client; derive per-protocol
  default (sing-box / mihomo / v2rayN) from source_type or flag
- config: default user_agent to empty so the per-protocol default kicks
  in; non-empty config value still wins as an explicit override

Author: bitxwave

src/core/config.rs

@@ -49,7 +49,10 @@ pub struct AppConfig {
 }
 
 fn default_user_agent() -> String {
-    "Mozilla/5.0 (compatible; ProxyConvert/2.0)".to_string()
+    // Empty by default -> SourceLoader picks a protocol-matched UA per-request
+    // (sing-box/mihomo/v2rayN). Subscription panels route by UA, so a generic
+    // UA like "Mozilla/... ProxyConvert/..." gets rejected.
+    String::new()
 }
 
 fn default_timeout() -> u64 {
@@ -229,10 +232,7 @@ mod tests {
     fn test_default_config() {
         let config = AppConfig::default();
 
-        assert_eq!(
-            config.user_agent,
-            "Mozilla/5.0 (compatible; ProxyConvert/2.0)"
-        );
+        assert_eq!(config.user_agent, "");
         assert_eq!(config.timeout_seconds, 30);
         assert_eq!(config.retry_count, 3);
         assert_eq!(config.log_level, "info");

src/utils/source/loader.rs

@@ -46,14 +46,43 @@ impl SourceLoader {
             // Use source flag if set (empty = &flag=), else protocol default
             let url_with_flag =
                 Self::append_flag_to_url(source, &source_meta.source_type, source_meta.flag.as_deref());
-            Self::load_from_url(&url_with_flag, config).await
+            // Pick a User-Agent that subscription panels recognize.
+            // Why: v2board/xboard/sspanel-style panels route responses by UA;
+            // the default reqwest UA is rejected or silently dropped, surfacing
+            // as "address unreachable" even when the host is reachable.
+            let ua = Self::effective_user_agent(&source_meta.source_type, source_meta.flag.as_deref(), config);
+            Self::load_from_url(&url_with_flag, &ua, config).await
         } else {
             // File path: use only the part before ? (query params are kept in source string for reference)
             let path = source.find('?').map(|i| &source[..i]).unwrap_or(source.as_str());
             Self::load_from_file(path)
         }
     }
 
+    /// Choose the User-Agent to send with subscription requests.
+    /// Precedence: explicit `config.user_agent` (non-empty) > protocol-matched default.
+    fn effective_user_agent(
+        source_type: &Protocol,
+        flag_override: Option<&str>,
+        config: &AppConfig,
+    ) -> String {
+        let ua = config.user_agent.trim();
+        if !ua.is_empty() {
+            return ua.to_string();
+        }
+        // Derive from flag if the user overrode it, otherwise from source_type.
+        let kind = flag_override
+            .and_then(Protocol::from_str)
+            .unwrap_or(*source_type);
+        // Name-only; subscription panels typically match on the keyword, not the version.
+        // Users who hit a version-strict panel can override via config.user_agent.
+        match kind {
+            Protocol::SingBox => "sing-box".to_string(),
+            Protocol::Clash => "mihomo".to_string(),
+            Protocol::V2Ray => "v2rayN".to_string(),
+        }
+    }
+
     /// Append or update flag query parameter to URL.
     /// Use flag_override if set (empty string = &flag=), else source_type default.
     fn append_flag_to_url(
@@ -151,11 +180,12 @@ impl SourceLoader {
     }
 
     /// Load content from URL (uses NetworkError for fetch failures).
-    async fn load_from_url(url: &str, config: &AppConfig) -> Result<String> {
-        tracing::info!("Fetching URL: {}", url);
+    async fn load_from_url(url: &str, user_agent: &str, config: &AppConfig) -> Result<String> {
+        tracing::info!("Fetching URL: {} (UA: {})", url, user_agent);
 
         let client = reqwest::Client::builder()
             .timeout(std::time::Duration::from_secs(config.timeout_seconds))
+            .user_agent(user_agent)
             .build()
             .map_err(|e| ConvertError::network_error(e.to_string().as_str()))?;