fix: [Studio] setup.ps1 update-flow for windows

[rolandtannous]

Summary

• Ports the update-flow logic from setup.sh (introduced in Consolidate dual venvs and separate install from update #4530) to setup.ps1
• unsloth studio update on Windows now checks installed version against PyPI latest and skips all Python dependency work (PyTorch, install_python_stack.py, transformers 5.x) when already up-to-date
• Falls through to full install if PyPI is unreachable (safe default)
• Footer banner shows "Unsloth Studio Updated" vs "Unsloth Studio Setup Complete" depending on context

Test plan

Requires a Windows machine with an existing unsloth studio install (~\.unsloth\studio\unsloth_studio venv).

Fast path (versions match)

• Run unsloth studio update when installed unsloth version matches PyPI latest
• Verify output shows unsloth X.Y.Z is up to date and dependencies up to date
• Verify PyTorch is NOT re-downloaded (~2.8 GB saved)
• Verify footer says "Unsloth Studio Updated"

Update path (new version available)

Since downgrading unsloth also replaces setup.ps1 in site-packages with the old version (no version check), use this workaround:

```powershell

1. Install from this branch

./install.ps1 --local

2. Downgrade unsloth inside the venv

& "$env:USERPROFILE.unsloth\studio\unsloth_studio\Scripts\pip" install unsloth==2026.3.16

3. Replace setup.ps1 with the new version from this branch

$setupPath = & "$env:USERPROFILE.unsloth\studio\unsloth_studio\Scripts\python" -c "import studio; import pathlib; print(pathlib.Path(studio.file).parent / 'setup.ps1')"
Invoke-WebRequest -Uri "https://raw.githubusercontent.com/unslothai/unsloth/fix/setup-ps1-update-flow/studio/setup.ps1" -OutFile $setupPath

4. Run update — should detect version mismatch and do a full install

unsloth studio update
```

• Verify output shows unsloth 2026.3.14 -> X.Y.Z available, updating...
• Verify full dependency install runs

Install path (called from install.ps1)

• Set $env:SKIP_STUDIO_BASE = "1" and run setup.ps1
• Verify version check is skipped, full install runs as before
• Verify footer says "Unsloth Studio Setup Complete"

Offline/unreachable PyPI

• Disconnect network or set a bogus proxy, run unsloth studio update
• Verify output shows could not reach PyPI, updating to be safe...
• Verify full dependency install runs (safe fallback)

[gemini-code-assist]

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[danielhanchen]

Thank you for the PR! The goal of this PR is to port the update-flow fast-path from setup.sh to setup.ps1 for Windows, so unsloth studio update can skip redundant Python dependency work when the installed version matches PyPI. As a summary, this PR adds PyPI version checking to setup.ps1, moves pip upgrade behind the fast-path gate, changes the footer label to distinguish setup vs update, and clears stale session env vars (SKIP_STUDIO_BASE, STUDIO_LOCAL_REPO) in the CLI update() function.

I merged main into the branch (no conflicts -- behind-main commits touched unrelated export UI files) and applied three fixes on top:

1. $InstalledVer normalization -- wrapped the python -c capture with | Out-String).Trim() to handle PowerShell 5.1's array-vs-scalar output behavior. Also added .Trim() to $LatestVer for defensive parity.
2. Moved Fast-Install --upgrade pip inside if (-not $SkipPythonDeps) -- the fast path was still doing a network round-trip to upgrade pip before deciding to skip everything else.
3. STUDIO_LOCAL_REPO env cleanup -- update() now pops STUDIO_LOCAL_REPO when --local is not passed, preventing a stale --local session from leaking into a plain update.

Review summary (3 rounds, 24 independent reviewers)

Reviewers	Severity	Finding
4/24	Low	Footer uses SKIP_STUDIO_BASE to infer setup vs update; direct setup.ps1 or unsloth studio setup runs show "Updated" instead of "Installed". Intentional per PR description.
3/24	Info	Fast path only checks unsloth version, not unsloth-zoo -- same as setup.sh behavior.
2/24	Info	Fast path does not validate .venv_t5 presence -- same as setup.sh behavior.
2/24	Info	$_PkgName injection into Python -c string -- same pattern as setup.sh, user controls their own --package flag.
1/24	Info	UNSLOTH_NO_TORCH can leak from install.ps1 --no-torch session -- pre-existing issue in main, not introduced by this PR.

Concrete suggestions for each finding below.

[danielhanchen]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a fast-update path for Python dependencies in the PowerShell setup script by comparing the currently installed version against the latest version on PyPI. It also improves environment variable management in the CLI update command to prevent state inheritance issues. A review comment suggests renaming the variable $_PkgName to avoid potential confusion with PowerShell's automatic pipeline variable $_.

[gemini-code-assist]

In PowerShell, $_ is an automatic variable that represents the current object in the pipeline. Using it as part of a variable name like $_PkgName is unconventional and can be confusing for other developers. It's better to use a name that doesn't include special automatic variables.

I suggest renaming $_PkgName to $pkgName for clarity and adherence to PowerShell best practices. This change should be applied to all occurrences of $_PkgName in this block (lines 1362, 1365, 1370, 1373).

$pkgName = if ($env:STUDIO_PACKAGE_NAME) { $env:STUDIO_PACKAGE_NAME } else { "unsloth" }