docs: remove banner and restore gemini info in README

Author: rithythul

README.md

@@ -1,7 +1,3 @@
-<div align="center">
-<img width="1200" height="475" alt="GHBanner" src="https://github.com/user-attachments/assets/0aa67016-6eaf-458a-adb2-6e31a0763ed6" />
-</div>
-
 # KOOMPI TYPING
 
 Master Khmer & English typing through gamified adventures. Build your streak and level up!
@@ -32,11 +28,22 @@ bun run build
 bun run preview
 ```
 
+## Environment Variables
+
+Create a `.env.local` file for optional AI-powered text generation:
+
+```bash
+GEMINI_API_KEY=your_api_key_here
+```
+
+> **Note:** The app works fully offline without an API key using built-in typing content.
+
 ## Tech Stack
 
 - ⚛️ React 19 + TypeScript
 - 🎨 Tailwind CSS v4
 - ⚡ Vite
+- 🤖 Google Gemini (optional, for AI-generated practice texts)
 
 ## License