feat: add unified reverse proxy URL support for all API endpoints

Extract URL construction into src/utils/url-builder.js, replacing 8
scattered inline implementations. Now properly handles reverse proxy
paths like /Gemini/v1 or /antigravity/v1 by detecting version segments
at any path depth. Fixes inconsistent Anthropic URL in multi-ai-generator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/index.js

@@ -27,3 +27,10 @@ export {
     reloadKeywordsPromptTemplate,
     reloadHistoricalPromptTemplate,
 } from './prompt-template';
+
+export {
+    buildOpenAIChatUrl,
+    buildOpenAIModelsUrl,
+    buildAnthropicUrl,
+    buildGoogleUrl,
+} from './url-builder';