[SandObserver]

Summary

• Add ui/js/label-contrast.js: WCAG relative luminance and contrast, cover source mapping, per-cell luminance from sampled pixels, and the rect-to-cell lookup that picks a label tone.
• Pick the tone by worst-case contrast across the cells a label covers, so a label spanning light and dark takes the colour that holds up on both.
• Sample the wallpaper through a separate crossOrigin image into a 32x18 grid, with the rendered --bg-brightness folded into each channel; a tainted or unreachable read leaves the labels untoned.
• Tone solid-colour backgrounds from the parsed colour, resolved through a two-seed canvas so an unparseable value is not read back as the seed.
• Re-tone on layout build, page rebuild and orientation change, and re-sample on resize, since cover re-maps the image to the new viewport.
• Measure each label against its own page's origin rather than the viewport, so the pages waiting off-screen are toned for where they will land.
• Skip labels inside a folder overlay: that scrim replaces the background.
• Add [data-tone="dark"] label colours in both the default and increased-contrast themes, with a hairline shadow instead of the heavy one.

Testing
All green, including the Docker build. Added 12 frontend tests for luminance, contrast, cover mapping, brightness, cell lookup and the mixed-patch tone choice.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.