Say where background effects run slowly

- Where the browser has no MediaStreamTrackProcessor, effects draw each
  frame through a canvas: slower, and the first build stalls the page.
  They stay offered and choosable there, with a notice under the grid
  that says so and describes the group.
- Decided in the view model, beside whether effects can run at all, so
  the lobby and the call give the same answer.
- The notice stays within the menu's width however long the sentence,
  which the story checks.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/components/CallFooter.tsx

@@ -86,7 +86,7 @@ export interface FooterState {
   /** Every effect on offer, in the order they are shown. */
   backgroundEffects: BackgroundEffectChoice[];
   /** What the user is told about effects here, if anything. */
-  backgroundEffectNotice: "unavailable" | undefined;
+  backgroundEffectNotice: "unavailable" | "slow" | undefined;
   showFooter: boolean;
 
   /* This is needed for WindowMode = "flat" */
@@ -243,7 +243,9 @@ export const CallFooter: FC<FooterProps> = ({
         backgroundEffectNotice={
           backgroundEffectNotice === "unavailable"
             ? t("background_effects.unavailable")
-            : undefined
+            : backgroundEffectNotice === "slow"
+              ? t("background_effects.slow")
+              : undefined
         }
       />,
     );