fix(deepseek): drop /v1 from probe URL

Matches the arcadia-app providers map update — direct-mode "Test
connection" was probing https://api.deepseek.com/v1/models which 404s
on DeepSeek's new endpoint. Now probes /models at the host root.

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

app/routes/settings.tsx

@@ -120,7 +120,7 @@ export default function SettingsRoute() {
             : s.providerId === "openai"
               ? "https://api.openai.com/v1"
               : s.providerId === "deepseek"
-                ? "https://api.deepseek.com/v1"
+                ? "https://api.deepseek.com"
                 : "https://dashscope-intl.aliyuncs.com/compatible-mode/v1")
         // Resolve key for the probe (lmstudio doesn't need one).
         let apiKey: string | undefined