Name the background effect pieces after background effects

- One name, as review asked: TrackProcessorContext becomes
  BackgroundEffectsContext, with BackgroundEffectsProvider,
  BackgroundEffectsState, useBackgroundEffects,
  useBackgroundEffectsState$ and useSyncBackgroundEffects.
- The state's Behavior is backgroundEffectsState$ wherever it is passed,
  in place of trackProcessorState$, trackerProcessorState$ and
  processorState$.
- Renames only: nothing changes in behaviour.

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

src/settings/SettingsModal.tsx

@@ -28,7 +28,7 @@ import {
 import { PreferencesSettingsTab } from "./PreferencesSettingsTab";
 import { Slider } from "../Slider";
 import { DeviceSelection } from "./DeviceSelection";
-import { useTrackProcessor } from "../livekit/TrackProcessorContext";
+import { useBackgroundEffects } from "../livekit/BackgroundEffectsContext";
 import {
   DeveloperSettingsTab,
   type DeveloperSettingsSnapshot,
@@ -81,7 +81,7 @@ export const SettingsModal: FC<Props> = ({
 
   // Generate a `Checkbox` input to turn blur on or off.
   const BlurCheckbox: React.FC = (): ReactNode => {
-    const { supported } = useTrackProcessor();
+    const { supported } = useBackgroundEffects();
 
     const [blurActive, setBlurActive] = useSetting(backgroundBlurSetting);