Add gemini-prompt.html - long context prompt UI for Gemini models (#229)

* 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

* Set gemini-3-flash-preview as default model, list both gemini-3 models first

https://claude.ai/code/session_018mtnPjdfk2xNeu1nncobW9

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Co-authored-by: Claude <noreply@anthropic.com>

Author: simonw