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/lib/api/model-fetch.ts

@@ -18,12 +18,14 @@ export async function fetchModelsForConfig(
   apiKey: string,
   isFullUrl?: boolean,
   modelsUrl?: string,
+  customUserAgent?: string,
 ): Promise<FetchedModel[]> {
   return invoke("fetch_models_for_config", {
     baseUrl,
     apiKey,
     isFullUrl,
     modelsUrl,
+    customUserAgent,
   });
 }