fix: support custom base URL for Gemini provider (#520)

* fix: support custom base URL for Gemini provider

Fixes #512

The Gemini provider was throwing an error when a custom base URL was set,
causing the chat interface to crash. The @google/genai SDK actually supports
custom base URLs via the httpOptions.baseUrl configuration.

This change removes the error and passes the custom base URL to the SDK,
enabling users to use custom Gemini-compatible endpoints (like proxies or
self-hosted alternatives).

* fix: Fix lint

Author: kevin-on

src/core/llm/gemini.ts

@@ -49,11 +49,13 @@ export class GeminiProvider extends BaseLLMProvider<
 
   constructor(provider: Extract<LLMProvider, { type: 'gemini' }>) {
     super(provider)
-    if (provider.baseUrl) {
-      throw new Error('Gemini does not support custom base URL')
-    }
 
-    this.client = new GoogleGenAI({ apiKey: provider.apiKey ?? '' })
+    this.client = new GoogleGenAI({
+      apiKey: provider.apiKey ?? '',
+      httpOptions: provider.baseUrl
+        ? { baseUrl: provider.baseUrl.replace(/\/+$/, '') }
+        : undefined,
+    })
     this.apiKey = provider.apiKey ?? ''
   }