Replace manual @load/@error handlers and imageError ref with useImage
from @vueuse/core. Source is wrapped in computed() so useImage re-runs
when imageUrl changes.
Cache the last successfully loaded naturalWidth/Height in refs that
update only when isReady fires. This avoids the placeholder text
flickering back to 'Calculating dimensions' each time imageUrl changes
during live preview streaming.
Test mocks useImage at module level and drives state/isReady/error
manually while preserving @testing-library/vue assertions.
84f0871277