feat: switch from OpenAI API to native Ollama Python client

This commit refactors the LLM integration to use Ollama's native Python client instead of OpenAI-compatible API, while fixing critical template syntax errors and improving project structure.

Key changes:
- Replace openai package with ollama package in backend requirements
- Rewrite llm.py to use ollama.AsyncClient for direct Ollama API calls
- Update main.py to use non-streaming Ollama responses with thinking extraction
- Fix template syntax error in MilkdownEditor.vue (GhostTextOverlay component tags)
- Fix string截取错误 by using slice() instead of substring()
- Add src/utils/api.js and src/utils/config.js for shared configuration
- Add CORS middleware to FastAPI backend
- Update prompt.py with clearer instructions for continuation generation
- Add comprehensive README.md documentation

BREAKING CHANGE: Environment variables OLLAMA_BASE_URL changed to OLLAMA_HOST (remove /v1/ suffix)

.env.example

@@ -0,0 +1,10 @@
+VITE_API_URL=http://localhost:8000/v1/completions
+
+# Ollama 配置
+OLLAMA_HOST=http://192.168.0.120:11434
+OLLAMA_MODEL=gpt-oss:120b
+
+# 可选：其他配置
+# 如果ollama需要认证，可以使用以下变量
+# OLLAMA_USERNAME=your_username
+# OLLAMA_PASSWORD=your_password