Add resume support for incomplete downloads

[dannyd40]

Summary

• Incomplete downloads can now be resumed instead of re-downloaded from scratch
• Button shows "⚠ Resume" (orange) when a local file is smaller than the remote file
• Clicking "Resume" downloads only the remaining bytes using HTTP Range requests
• Supports both multi-connection and single-connection resume depending on remaining size
• Backend accepts a new resume parameter in the /server_download/start endpoint

How it works

1. When a file exists locally, the frontend compares local vs remote size
2. If incomplete, the button becomes "⚠ Resume" instead of "✓ Downloaded"
3. On click, sends resume: true to the backend
4. Backend reads the existing file size and passes resume_from to the download function
5. Download function skips already-downloaded bytes and fetches only the remainder

Test plan

• Partially download a large model, then click "Resume" → should continue from where it stopped
• Fully downloaded model → should show "✓ Downloaded" (no resume button)
• Resume with server that supports Range requests → multi-connection resume
• Progress bar starts from the existing percentage, not 0%

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