fix: prevent blank GitHub widget previews

[bobleesj]

What problem does it solve?

GitHub and VS Code notebook exports could publish a blank WebGPU canvas because the exporter checked only that a canvas existed. The resulting preview hid the scientific image while leaving the surrounding widget controls visible.

What should reviewer(s) do?

Review the preview selection and pixel-content gate in src/quantem/widget/cli.py, then the focused blank-canvas and static-preview tests in tests/test_cli.py. The change affects quantem github export behavior only and does not change the public Python API.

Verification

• python -m pytest -q tests/test_cli.py: 74 passed with the source checkouts on PYTHONPATH.
• Six downstream Denova tutorial exports rendered nonblank scientific previews and retained unchanged source cells.
• The committed diff passed git diff --check and the outgoing privacy scrub.

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This PR follows the
scikit-package workflow for
reproducible scientific software.

[github-actions]

• Delete bobleesj/quantem.widget:fix/github-nonblank-previews from its remote if you own it, it is not long-lived, and no other PR uses it.
• Remove the dedicated worktree and local fix/github-nonblank-previews branch after confirming the worktree is clean.
• Fetch electronmicroscopy/quantem.widget and update local main before starting new work.