Add gemini-prompt.html - long context prompt UI for Gemini models

- System prompt textarea (small) with localStorage history
- Shows last 3 recent system prompts below textarea, truncated, clickable
- Expandable "N more…" link to reveal full history (up to 50 stored)
- Loads most recent system prompt on page load
- Large content textarea with Ctrl/Cmd+Enter submit shortcut
- Model selector with gemini-2.5-flash as default plus full model list
- API key loaded from GEMINI_API_KEY in localStorage or prompted
- Streaming response rendered as progressive markdown via marked.js
- Copy to clipboard button works during and after generation
- Token count and stream status shown below controls

https://claude.ai/code/session_018mtnPjdfk2xNeu1nncobW9

Author: claude