feat: dark mode fix, call wallpaper, setTheme error handling, Sentry filter

- CallEmbed: inject :root { color-scheme } into iframe so EC respects Cinny
  theme regardless of OS preference (fixes white background in dark mode)
- CallEmbed: store themeKind, update color-scheme CSS on live setTheme() calls
- CallEmbed: catch transport.send() rejection in setTheme() to prevent
  unhandled promise rejection when widget not ready yet (fixes REACT-8)
- CallEmbed: html + body both set to background:none so wallpaper shows through
- CallEmbedProvider: apply chatBackground wallpaper style to call embed
  container in full-view mode (not PiP) -- wallpapers carry over to calls
- useCallEmbed: pass themeKind through to CallEmbed constructor
- index.tsx: ignoreErrors: [Request timed out] to suppress matrixRTC
  heartbeat timeouts (REACT-9) from Sentry noise
- README: document 0.19.4, positioning fix, dark mode fix, wallpaper,
  millify Rolldown interop fix, Sentry noise filter

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/index.tsx

@@ -22,6 +22,8 @@ if (sentryDsn) {
     sendDefaultPii: false,
     // Forward Sentry logs to the dashboard
     enableLogs: true,
+    // Suppress benign PostmessageTransport / matrixRTC heartbeat timeouts (upstream library noise)
+    ignoreErrors: ['Request timed out'],
     beforeSend(event) {
       // Drop any event that may have leaked an access token into breadcrumbs/data
       if (JSON.stringify(event).includes('access_token')) return null;