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>

pnpm-lock.yaml

@@ -163,6 +163,9 @@ importers:
       eslint-plugin-storybook:
         specifier: ^10.3.6
         version: 10.6.0(eslint@8.57.1(supports-color@7.2.0))(supports-color@7.2.0)(typescript@5.9.3)
+      fake-indexeddb:
+        specifier: ^6.2.5
+        version: 6.2.5
       fetch-mock:
         specifier: 11.1.5
         version: 11.1.5(node-fetch@2.7.0)
@@ -3998,6 +4001,10 @@ packages:
     resolution: {integrity: sha512-KfYbmpRm0VbLjEvVa9yGwCi9GI34xvi7A/HXYWQO65CSD2u3MczUJSuwXKFIxlGsgBQizV9q5J9NHj4VG0n+pA==}
     engines: {node: '>=12.0.0'}
 
+  fake-indexeddb@6.2.5:
+    resolution: {integrity: sha512-CGnyrvbhPlWYMngksqrSSUT1BAVP49dZocrHuK0SvtR0D5TMs5wP0o3j7jexDJW01KSadjBp1M/71o/KR3nD1w==}
+    engines: {node: '>=18'}
+
   fast-deep-equal@3.1.3:
     resolution: {integrity: sha512-f3qQ9oQy9j2AhBe/H9VC91wLmKBCCU/gDOnKNAYG5hswO7BLKj09Hc5HYNz9cGI++xlpDCIgDaitVs03ATR84Q==}
 
@@ -9548,6 +9555,8 @@ snapshots:
 
   expect-type@1.4.0: {}
 
+  fake-indexeddb@6.2.5: {}
+
   fast-deep-equal@3.1.3: {}
 
   fast-glob@3.3.3: