Say why a file cannot be used as a background

- The reason appears with the grid, where the user chose the file, using
  the same Alert the reaction menu already reports errors with
- A reason for each: not an image, animated, could not be opened, and a
  fallback for anything the store itself refuses
- Named one by one rather than built from the reason, so the extractor
  finds every string it has to translate
- The background in force is untouched, which is the other half of
  FR-021: being refused a file changes nothing
- Covered by a story, and checked end to end by offering a text file:
  the message appears, the menu stays open, nothing is kept

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

locales/en/app.json

@@ -96,6 +96,10 @@
   },
   "disconnected_banner": "Connectivity to the server has been lost.",
   "error": {
+    "background_animated": "Animated images cannot be used as a background",
+    "background_not_an_image": "That file is not an image",
+    "background_not_kept": "That background could not be saved",
+    "background_undecodable": "That image could not be opened",
     "call_is_not_supported": "Call is not supported",
     "call_not_found": "Call not found",
     "call_not_found_description": "<0>That link doesn't appear to belong to any existing call. Check that you have the right link, or <2>create a new one</2>.</0>",