Improve image handling: auto-resize toggle, SVG support, and DOM parsing fix

What changed

• Advanced setting: Auto Resize (auto_resize). When disabled, img src is set directly to the Gumlet URL instead of the placeholder pixel (no viewport-based resize via gumlet.js for those tags).
• Fix DOM parsing for img/source tags: do not entity-encode the whole tag before loadHTML (was preventing any img from being found).
• Support SVG URLs in replace_image_url.
• Normalize JSON-style escapes in fragments (wp_unslash, /) before parsing.