1. Loading text: shows "Loading model..." for cached models,
"Downloading model..." for new downloads. Toast description
adapts accordingly.
2. Download progress: polls /api/models/gguf-download-progress every
2s during downloads, updating the toast with percentage and GB
downloaded. Progress is estimated by checking the HF cache folder
size against the expected total bytes.
3. Passes isDownloaded and expectedBytes through the full chain from
variant click to selectModel for accurate UI state.
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