Model Manager UI: download, switch, and update models from the desktop app

[kovtcharov]

Problem

Users must use the CLI or manually manage GGUF files to download, switch, or update models. There is no graphical interface for model management. When Lemonade is downloading a model, the UI shows a spinner with no progress (#588 covers partial feedback).

Strategic Context

From the OpenClaw strategy (Track B Consumer UX):

"Model manager: Download, update, and switch models from the UI. Show model size, hardware requirements, and recommended use cases. Users should never need to understand GGUF files or run CLI download commands."

This is table stakes for non-technical users. OpenClaw users rely on Ollama's model library UI. GAIA needs an equivalent.

Proposed Features

1. Model library — Browse available models with size, quality rating, and hardware requirements
2. One-click download — Download button with progress bar, speed, ETA
3. Model switching — Switch active model from dropdown, hot-swap without restart
4. Hardware recommendations — "Recommended for your hardware" badges per model
5. Storage management — Show disk usage per model, delete unused models
6. Update notifications — Alert when newer model versions are available

Acceptance Criteria

• User can browse and download models from the Agent UI
• Download progress with percentage, speed, and ETA
• Switch active model without CLI
• Models show hardware compatibility info
• Disk usage visible per model