lotus(#5): native transparent-background + Lotus theme hooks

Adds body.lotus-transparent (lotusTransparent=1) so the host wallpaper
shows through the call natively, retiring cinny's injected
`html,body{background:none!important}` hack; and body.lotus-theme
(lotusTheme=1) as a source-level Compound-token override hook for the
Lotus/TDS palette (driven by the existing setTheme channel / URL flags).
Additive: no-op without the flags.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/useTheme.ts

@@ -11,6 +11,7 @@ import { type IThemeChangeActionRequest } from "matrix-widget-api";
 
 import { getUrlParams } from "./UrlParams";
 import { widget } from "./widget";
+import { lotusFlag } from "./lotus/lotusWidget";
 
 export const useTheme = (): void => {
   const [requestedTheme, setRequestedTheme] = useState(
@@ -57,5 +58,10 @@ export const useTheme = (): void => {
       previousTheme.current = themeString;
     }
     document.body.classList.remove("no-theme");
+    // [lotus #5] Native theming hooks, opted in by the host via URL flags, so
+    // it no longer has to inject CSS into the iframe after load.
+    if (lotusFlag("lotusTransparent"))
+      document.body.classList.add("lotus-transparent");
+    if (lotusFlag("lotusTheme")) document.body.classList.add("lotus-theme");
   }, [previousTheme, requestedTheme]);
 };