Introduces src/ui/AssetPicker.tsx, a single component that replaces the
~6 copy-pasted image-source UIs (upload/browse/URL) with one that also
supports video via `mediaType`. Full variant mirrors ImageStylePanel's
existing look; compact variant is a tight single row for array-editor
item cards. Both share all state/handler logic, built on the C1
uploadAsset/listAssets util.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
