fix(gemini): Use specific selector to avoid empty export with thoughts
The Gemini-to-Markdown userscript was incorrectly exporting an empty response when the "Show thoughts" section was expanded. This was because the script used a generic .markdown selector, which would select the "thoughts" panel first when it was visible.
This change modifies the selector to the more specific .model-response-text to ensure that only the main response content is selected, regardless of whether the "thoughts" panel is open or closed.
Fix Gemini Empty Export with Thoughts（Merge pull request #50 from Aiuanyu/fix-gemini-empty-export-with-thoughts-9939315440809774499）
