feat(ui): add Basic Chat interface for testing models

Add a simple chat UI to the dashboard for quickly testing AI models from
connected providers. Features include:
- Model picker from all connected providers
- Streaming chat responses
- Image attachment support
- Session history with localStorage persistence
- Responsive design with dark theme

Note: Removed build.sh from original PR as it contained syntax errors and
was unrelated to the chat UI feature.

Co-authored-by: Nguyễn Trung Hiếu <140531897+bonelag@users.noreply.github.com>
Made-with: Cursor

src/shared/components/Sidebar.js

@@ -13,6 +13,7 @@ const navItems = [
   { href: "/dashboard/endpoint", label: "Endpoint", icon: "api" },
   { href: "/dashboard/providers", label: "Providers", icon: "dns" },
   { href: "/dashboard/proxy-pools", label: "Proxy Pools", icon: "lan" },
+  { href: "/dashboard/basic-chat", label: "Basic Chat", icon: "chat" },
   { href: "/dashboard/combos", label: "Combos", icon: "layers" },
   { href: "/dashboard/usage", label: "Usage", icon: "bar_chart" },
   { href: "/dashboard/quota", label: "Quota Tracker", icon: "data_usage" },