ui: support a light theme in the panel

[erkamyaman]

The panel is dark only (app/src/styles/_theme.scss sets color-scheme: dark from one palette). People who use light browser developer tools get a dark panel in a light window. Split out of #181, which settled on dark only for now and removed the light-scheme checks from the contributor docs.

Proposed scope

• Add a light neutral palette under prefers-color-scheme: light, with a data-theme override.
• Pass the theme from extension/devtools.js to the panel with chrome.devtools.panels.themeName, and update it with setThemeChangeHandler: https://developer.chrome.com/docs/extensions/reference/api/devtools/panels#property-themeName
• Remove the hardcoded dark values in extension/panel.html, extension/ui/index.html, app/index.html and app/src/hub-rail-style.ts.
• Check accent and status color contrast on light surfaces against WCAG AA.
• Put the light scheme back in scripts/panel-axe.mjs, the devtools-verify skill, the a11y-reviewer role, CONTRIBUTING.md and the development docs.