Decode portal wallpapers by content and simplify status bar

Detect wallpaper image formats from their contents so extensionless portal copies render on the desktop and lock screen. Preserve decode limits and cover extensionless files with regression tests.

Remove the Bluetooth icon from the status bar.

compositor/Cargo.toml

@@ -26,3 +26,6 @@ serde_json.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 xcursor.workspace = true
+
+[dev-dependencies]
+tempfile.workspace = true