feat(site): support main-site embedding and clarify Scans navigation

[mstrhakr]

Summary

Support embedding Scans inside the main website's shared header/footer while keeping the standalone site focused on Scans.

• Activate headerless dark-mode embedding only after an origin- and source-checked handshake with the immediate parent at https://printmaster.work.
• Report content height and anchor positions so the main-site wrapper can resize and scroll without nested navigation.
• Preserve standalone navigation/theme behavior, including direct visits with ?embed=1 when there is no trusted parent handshake.
• Clarify standalone navigation: local setup/release links, explicitly labeled external Scans resources, and a separate "More from PrintMaster" footer for fleet monitoring and Agent/Server downloads.
• Add embed regression tests to CI and document hosting/deployment constraints.

Validation

• After rebase: node --test .\tests\site.test.cjs .\tests\embed.test.cjs — 9 tests passed.
• Browser integration previously verified responsive iframe sizing, copy buttons, outer-page section navigation, matching dark theme, and standalone light/dark behavior with local code intercepted at the production origins.

Companion and deployment

The companion Printmaster-Org/printmaster.work commit 503e7eb adds the /scans/ wrapper with shared navigation, iframe messaging validation, clipboard delegation, and a standalone fallback. It is separate from this PR.

Deploy this Scans support first, then the main-site wrapper. After merge and successful main CI, refresh Pages from main with the currently promoted published stable version. No new Windows utility release is needed.

GitHub Pages cannot configure a custom HTTP frame-ancestors policy. The handshake restricts activation of embedded mode, not who can frame the public standalone site; the README documents an edge/server policy for hosting that supports it.