Report unreadable cropper sources with the loader's error codes

[codenameakshay]

When systemCropper got a content URI that no longer exists, it returned failed — system-ui-failed — Unable to open the wallpaper cropper. Android resolves the crop intent by the URI's MIME type. getType returns null for a missing row, the intent lookup finds nothing, and getCropAndSetWallpaperIntent throws.

 openSystemCropper(uri)
+  getType(uri) == null       → failed · source-unavailable
+  getType(uri) not image/*   → failed · invalid-content-type
   getCropAndSetWallpaperIntent(uri) → startActivity → previewOpened

These are the same codes the direct path already uses for the same URI.

Verification

• API 36 emulator:
  • content://media/external/images/media/1 (missing) → failed — source-unavailable — The content URI could not be read., with the same result for Home and Lock.
  • A downloaded MediaStore image still opens WallpaperCropActivity → previewOpened.
• Tests: a JVM test covers the MIME-type mapping.
• Local CI: pass.

🤖 Generated with Claude Code