HTML emails with black text became unreadable when viewed in dark mode because the WebView inherited a dark background from the system theme. Inject `color-scheme: light` CSS + meta tag so the WebView always renders email content on a white background, regardless of the device theme. Extracts `buildEmailHtml()` as a `@visibleForTesting` top-level function and adds unit tests to cover the light-mode enforcement and CSP logic. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>