feat: Updated screensaver image mode fetches random images (#2430)

BREAKING CHANGE: The `screensaver` image mode now shows a random image
from [picsum.photos](picsum.photos) instead of the embedded default
image. If you previously set `image.mode: screensaver`, change it to
`image.mode: default` to restore the previous behavior.

src/localize/languages/en.json

@@ -298,8 +298,9 @@
         "mode": "Image mode",
         "modes": {
           "camera": "Home Assistant camera snapshot of camera entity",
+          "default": "Default embedded image",
           "entity": "Entity with entity_picture attribute",
-          "screensaver": "Embedded screensaver image",
+          "screensaver": "Random image",
           "url": "Arbitrary image specified by URL"
         },
         "proxy": {