Install freezes mid-progress ("70%") for first-time Windows users

Source: Typeform feedback from a first-time Windows user (davidpolcino@gmail.com) on v1.0.9:

> "I don't know because the desktop install freezes at 70%"
> "Having the install finish would be nice :-)"

## Triage so far

Install progress in `src/main/sources/standalone/install.ts` is fluid (file-copy %), not a hard-coded step at 70%. "Freezes at 70%" most likely means the **Copying packages…** phase stalls mid-way. The progress text at that point reads:

`Copying packages… N / M files · X elapsed · Y remaining`

## Most likely root cause (Windows-specific)

**Windows Defender (or third-party AV) scanning each Python package file individually as it's copied.** First-time users have no Defender exclusion for the install path, so every `.pyd` / `.dll` / `.so` lands → scanned → released → next file. For large packages (torch, numpy) this can hang for many minutes per file and feel like a freeze.

## Other possibilities (in descending probability)

1. Antivirus scanning (above)
2. SSD with low free space + low write cache
3. File-system quirk (network drive, encrypted volume, OneDrive folder)
4. Specific package file failing silently
5. Bootstrap-python phase blocking (less likely — that's earlier in the pipeline)

## Suggested actions

- [ ] Reach out to David (he left his email) for his `comfyui.log` + `main.log` to confirm
- [ ] Add a Windows-specific install pre-flight: detect `%LOCALAPPDATA%\Programs\ComfyUI` is under Defender protection and either (a) prompt to add an exclusion or (b) warn user that install may take 10–20 min
- [ ] Add UI affordance: if file-copy stalls > 30s without progress, surface a "This is taking a while — see common causes" tooltip linking to docs

Day-3 launch impact: 30k DAU but unclear how many first-time installs are bouncing because they assume it's broken.