feat: added a welcome msg for local LLM users

public/chat/welcome_local_llm.md

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+## Local LLM Mode
+
+Welcome to **K.G.Studio Musician Assistant** in local LLM mode.
+
+- No external API calls are required. Everything runs directly in your browser, with no extra API cost.
+- This mode uses **Gemma 4 E4B** through **LiteRT-LM** with **WebGPU** acceleration.
+- Recommended hardware: a GPU with at least **8 GB VRAM** or a system with at least **16 GB unified RAM**.
+- Performance is more limited than larger cloud-hosted models, especially on harder planning, editing, and multi-step tasks.
+
+### Recommended Workflow
+- Keep requests small and focused.
+- Guide the model step by step toward the final goal.
+- Work on smaller music regions instead of large full-song edits.
+- Prefer simpler music arrangements when possible.
+- Start a new conversation for each standalone task.
+
+### Use an External LLM Instead
+- If you want a larger cloud or self-hosted model, open **Settings -> General -> LLM Provider** and switch away from **Local LLM (Browser)**.
+- For a cloud model, you can use **OpenAI**, or choose **OpenAI Compatible** and enter a provider such as OpenRouter.
+- For a self-hosted model, choose **OpenAI Compatible** and enter your server's **Base URL** and **Model**.
+- After switching providers, start a new conversation so the chat uses the new model cleanly.