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>

locales/en/app.json

@@ -26,6 +26,7 @@
     "blur": "Blur",
     "none": "None",
     "numbered": "Background {{n}}",
+    "slow": "Background effects run slowly on this platform, which may cause your video to stutter.",
     "unavailable": "Background effects are not supported on this platform."
   },
   "call_ended_view": {