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>

package.json

@@ -94,6 +94,7 @@
     "copy-to-clipboard": "^3.3.3",
     "eslint-plugin-element-call": "link:eslint",
     "eslint-plugin-storybook": "^10.3.6",
+    "fake-indexeddb": "^6.2.5",
     "fetch-mock": "11.1.5",
     "i18next": "^26.0.0",
     "i18next-browser-languagedetector": "^8.0.0",