Refactor: 重构会话和模型控制区域样式，优化按钮和状态文本布局

Refactor UI: Group session tabs with model selector

Refine CSS positioning for improved layout and context

Force static positioning for session container

Modularize frontend CSS and JavaScript assets

Feat: Redesign session management to use tabs

Based on the provided git diff, here's the concise and descriptive commit message:

修复图片显示问题：移除红框并添加后端图片代理

Refactor: Improve input area layout and update model options

Refactor: Improve input area layout and button styling

Refine input area and button styles

feat: Add styles for model information section

docs: Add Gemini model tier details and limitations

Refactor main content layout to vertical flexbox

feat: Refactor Gemini proxy with native types; add Chinese license

Refactor: Improve UI layout and prepare metrics for extensibility

Refactor header styling; simplify login form.

Refine login page UI and user experience

feat: Implement multi-model selection and image input

Refactor: Modernize and enhance UI styling

Refactor: Enhance UI, improve documentation, and streamline setup

feat: Enhance UI layout and visual design

Initial project setup