Make session_id optional in chat endpoint - auto-create sessions

- Changed ChatMessage.session_id from required to Optional[str]
- Updated chat endpoint to generate UUID if session_id not provided
- Allows frontend to send messages without pre-creating a session
- Fixes 400 Bad Request errors when session_id is null

Author: smirk-dev

frontend/components/theme-provider.tsx

@@ -1,8 +1,7 @@
 'use client';
 
 import * as React from 'react';
-import { ThemeProvider as NextThemesProvider } from 'next-themes';
-import { type ThemeProviderProps } from 'next-themes/dist/types';
+import { ThemeProvider as NextThemesProvider, ThemeProviderProps } from 'next-themes';
 
 export function ThemeProvider({ children, ...props }: ThemeProviderProps) {
   return <NextThemesProvider {...props}>{children}</NextThemesProvider>;