feat(proxy): honor custom User-Agent across stream check and model fetch

Extract a shared `parse_custom_user_agent` helper in provider.rs returning
`Result<Option<HeaderValue>>`, and reuse it in the forwarder, stream check,
and model fetch paths so detection, forwarding, and model listing all apply
the same provider-level User-Agent. Previously only the forwarder honored it,
so stream check could fail (or model listing 403) on UA-gated upstreams that
the proxy itself handled fine.

- stream_check injects the provider's custom UA on the claude/codex paths and
  still skips the GitHub Copilot fingerprint UA.
- model_fetch service + command and the model-fetch.ts wrapper thread an
  optional UA through to GET /v1/models.
- runtime callers silently ignore invalid values via `.ok().flatten()`
  (no save-time block, so deeplink imports stay lenient).

src-tauri/src/commands/model_fetch.rs

@@ -14,12 +14,18 @@ pub async fn fetch_models_for_config(
     api_key: String,
     is_full_url: Option<bool>,
     models_url: Option<String>,
+    custom_user_agent: Option<String>,
 ) -> Result<Vec<FetchedModel>, String> {
+    // 与转发 / 检测路径共用 parse_custom_user_agent：非法 UA 静默忽略（不阻断取模型）。
+    let user_agent = crate::provider::parse_custom_user_agent(custom_user_agent.as_deref())
+        .ok()
+        .flatten();
     model_fetch::fetch_models(
         &base_url,
         &api_key,
         is_full_url.unwrap_or(false),
         models_url.as_deref(),
+        user_agent,
     )
     .await
 }