Add JSON output, validation, and screenshot capture

[next-devin]

This adds a stable machine interface to ntk: JSON on stdout, logs on stderr, per-file results, useful exit codes, quiet and no-progress modes, and ASCII progress outside a terminal.

It also:

• validates and normalizes store URLs
• makes Sass optional so core commands install on Windows ARM and current Python versions
• validates themes locally or through the server
• captures fixed desktop and mobile screenshots with Playwright
• tests the CLI on Ubuntu, Windows, and macOS ARM

Server validation depends on the Oscar endpoint requested in oscar-prime #5693.

This branch is based directly on master and does not include the retry work from #33. All 57 tests and flake8 pass locally. CI covers Python 3.10 through 3.14.

[next-devin]

Scope update: hosted screenshot infrastructure is out. This PR keeps deterministic screenshot capture local and in CI.

[codecov]

Codecov Report

❌ Patch coverage is 92.82700% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ntk/__main__.py	63.15%	7 Missing ⚠️
ntk/command.py	96.26%	4 Missing ⚠️
ntk/gateway.py	33.33%	4 Missing ⚠️
ntk/conf.py	92.85%	1 Missing ⚠️
ntk/utils.py	93.33%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[next-devin]

Current rollout: Theme Kit #34, Spark #39, and Skills #36. Oscar-owned work is tracked in NextCommerceCo/oscar-prime#5693. The developer-docs draft is closed until these contracts settle.

[next-devin]

Closing this draft after the 1.2.0 release. #36 keeps the smaller automation ideas open for maintainer feedback without rebasing this branch.