Extends the shared web/js/multi_loader.js to a third kind ("video") for
AI2GoMultiVideoLoader / AI2GoMultiVideoLoaderAdvanced: NODES map entries,
video/* MIME acceptance, drop-zone wording, a video-row preview (mm:ss +
WxH probed via a hidden <video>), and a serializeValue guard on force_rate
mirroring the existing max_size guard so a '' value can't fail validation.
README and the design spec's amendments section are extended to match.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
