Wire the background effects grid to the pipeline

- Footer view model exposes the chosen effect and a setter for it
- The footer names the catalogue: a view model holds no translated text
- The camera menu shows the grid in place of the blur toggle
- Shipped images are numbered, not named, since the art is a stand-in
- Withholding the setter is what renders the section disabled, as with
  the speaker section where no output can be chosen

Exploration for FEATURES_SPEC/2026-09_Background_Effects.md.

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

locales/en/app.json

@@ -4,6 +4,9 @@
   },
   "action": {
     "add_background_image": "Add image",
+    "background_effect_blur": "Blur",
+    "background_effect_none": "None",
+    "background_effect_numbered": "Background {{n}}",
     "blur_background": "Blur background",
     "close": "Close",
     "copy_link": "Copy link",