Add a background of one's own, kept on this device

- An add tile after the pictures opens the file picker, and the image
  chosen joins the backgrounds on offer without being put on; it is in
  force once chosen, like a shipped one. The tile is a command rather
  than a choice, so a menu item beside the radios, and it isn't offered
  where effects can't run.
- Kept in IndexedDB on this device and never sent anywhere, behind a
  small interface, so the unit tests use an in-memory store; the
  IndexedDB store itself is tested against fake-indexeddb, a new dev
  dependency.
- The picker takes the focus, which the menu reads as a click elsewhere,
  so the menu holds itself open until a file is chosen or the picker is
  cancelled.
- A chosen image the device no longer keeps, as after its storage is
  cleared, falls back to no effect once the device has said what it
  keeps.
- Switching is checked between any two kinds of effect. End to end on
  Chromium and Firefox: an added image shows in the preview once chosen;
  once it is in force, no request carries the file or the copy kept,
  which a deliberate upload fails; chosen, it is in force again after a
  reload, gone once storage is cleared, and can be added again.
- Kept as given here; what can be added is checked in the commit after.

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

src/components/CallFooter.tsx

@@ -68,6 +68,8 @@ export interface FooterActions {
   toggleVideo: (() => void) | undefined;
   /** Undefined where background effects can't be chosen. */
   selectBackgroundEffect: ((id: string) => void) | undefined;
+  /** Undefined where no background can be added. */
+  addBackgroundImage: ((file: File) => void) | undefined;
   toggleScreenSharing: (() => void) | undefined;
   /** Also controls if the settings button is visible */
   openSettings: (() => void) | undefined;
@@ -176,6 +178,7 @@ export const CallFooter: FC<FooterProps> = ({
   const selectBackgroundEffect = useBehavior(vm.selectBackgroundEffect$);
   const backgroundEffectNotice = useBehavior(vm.backgroundEffectNotice$);
   const backgroundEffectSettling = useBehavior(vm.backgroundEffectSettling$);
+  const addBackgroundImage = useBehavior(vm.addBackgroundImage$);
   const backgroundEffects = useBackgroundEffectLabels(
     useBehavior(vm.backgroundEffects$),
   );
@@ -244,6 +247,7 @@ export const CallFooter: FC<FooterProps> = ({
         selectedBackgroundEffect={backgroundEffect}
         onSelectBackgroundEffect={selectBackgroundEffect}
         backgroundEffectSettling={backgroundEffectSettling}
+        onAddBackgroundImage={addBackgroundImage}
         backgroundEffectNotice={
           backgroundEffectNotice === "unavailable"
             ? t("background_effects.unavailable")