Make browser login the default, and make modelslab update work on Windows and under package managers

[adhikjoshi]

A customer on Windows (pip, in a venv) ran modelslab auth login and got:

Email: serialkiller@gmail.com
Password:
Error: password: no input provided

Their account came from Continue with Google (signup_source = google), so it has no password. The prompt asked for something that does not exist, and the error never said so. The npm and PyPI READMEs send every new user to exactly that bare modelslab auth login.

Login

• Browser OAuth is the default in an interactive terminal. Email and password are still used with --email, --password or --browser=false, when stdin is not a terminal (scripts piping credentials), and when there is no local browser: over SSH, or Linux with no display. The callback listens on 127.0.0.1, which a browser across SSH cannot reach.
• An empty password now says no password entered and points at auth login --browser and auth forgot-password.

Update

Both the notification and modelslab update already existed. They had these problems:

Problem	Fix
modelslab update always failed on Windows ("Windows cannot replace a running executable")	Windows renames a running .exe; move it to .old, put the new one in place, delete .old on the next run
Under pip / pipx / uv / npm / Homebrew / Scoop it overwrote a file the package manager owns	Detect the install method from the binary path; print that manager's command instead
The notification told everyone to run modelslab update	Show the command for the install method, e.g. pip install --upgrade modelslab-cli
A failed check was not cached, so offline every command waited 1.5 s	Cache failures; keep what the last good check found
Notification printed in CI and into captured stderr	Skip when CI is set or stderr is not a terminal
Bare "permission denied" on /usr/local/bin or Program Files	Say to use sudo or an Administrator terminal

Verification

• go vet ./... and go test ./internal/... pass; 14 new tests (login mode, install-method detection, failed-check caching, replace, permission hint). Cross-builds for windows/amd64, windows/arm64, linux/amd64, darwin/amd64.
• Real binary, driven through a pty:
  • bare auth login → browser flow; over SSH / --browser=false / --email → password prompt; Enter at the prompt → the new hint.
• Real binary built as 0.1.4, against the live GitHub release:
  • notification shows modelslab update for a standalone copy and pip install --upgrade modelslab-cli for a copy under site-packages/modelslab_cli/bin/; silent with CI=1 and with redirected stderr;
  • modelslab update in the pip layout refuses with the pip command, exit 1;
  • the standalone copy self-updated 0.1.4 → 0.2.0, checksum verified, no .old/.new left.
• Not verified: the Windows rename path on a real Windows machine (cross-compiled only).

Note for the release

Users on 0.2.0 run the old updater until they upgrade, and its notice says modelslab update, which fails on Windows. Release notes and support replies should tell pip users to run pip install --upgrade modelslab-cli.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NJxK2YGapyVchfyE1F8CpR

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}