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>
Swap the flutter_html renderer for a webview_flutter-based widget that
enforces strict security by default: scripts blocked via CSP
(script-src 'none'), remote images opt-in, and every link click routed
through a confirmation dialog that bolds the registered domain for
phishing detection. Links open in the system browser via url_launcher.
On Linux (no webview_flutter platform support) the widget falls back to
plain text extracted via the existing htmlToPlain() utility.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
