fix(builder): safeImageUrl for FeaturesGrid/ContentSlider image sinks + tighten data:image allowlist

FeaturesGrid's <img src> and ContentSlider's CSS background-image url()
were still on safeUrl, which blocks data:image/svg+xml -- inconsistent
with other image sinks already swapped to safeImageUrl and a latent
regression for those two components. Swapped both to safeImageUrl;
left their navigation sinks (buttonUrl/buttonHref) on safeUrl.

Also tightened safeImageUrl's data:image allowlist check to require the
slash (dataimage/ not dataimage), so a bogus MIME like
data:imagehtml/... can no longer slip past the prefix check.

craft/src/components/sections/FeaturesGrid.tsx

@@ -1,7 +1,7 @@
 import React, { CSSProperties } from 'react';
 import { useNode, UserComponent } from '@craftjs/core';
 import { cssPropsToString } from '../../utils/style-helpers';
-import { escapeHtml, escapeAttr, safeUrl } from '../../utils/escape';
+import { escapeHtml, escapeAttr, safeUrl, safeImageUrl } from '../../utils/escape';
 
 interface FeatureItem {
   title: string;
@@ -116,7 +116,7 @@ FeaturesGrid.craft = {
   const idAttr = props.anchorId ? ` id="${escapeAttr(props.anchorId)}"` : '';
   const cards = (props.features || defaultFeatures).map((feat) => {
     const media = feat.image
-      ? `<img src="${escapeAttr(safeUrl(feat.image))}" alt="${escapeAttr(feat.imageAlt || feat.title || '')}" style="max-width:100%;height:auto;margin-bottom:16px;border-radius:8px">`
+      ? `<img src="${escapeAttr(safeImageUrl(feat.image))}" alt="${escapeAttr(feat.imageAlt || feat.title || '')}" style="max-width:100%;height:auto;margin-bottom:16px;border-radius:8px">`
       : `<div style="font-size:36px;margin-bottom:16px">${escapeHtml(feat.icon)}</div>`;
     const button = feat.buttonText
       ? `\n      <a href="${escapeAttr(safeUrl(feat.buttonUrl || '#'))}" style="display:inline-block;margin-top:16px;padding:10px 24px;background:#3b82f6;color:#fff;border-radius:8px;text-decoration:none;font-size:14px;font-weight:600">${escapeHtml(feat.buttonText)}</a>`