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/ContentSlider.toHtml.test.ts

@@ -109,6 +109,15 @@ describe('ContentSlider.toHtml renders slide.imageSrc as a background-image (INT
     expect(html).not.toContain('background-image:url(');
     expect(html).toContain('background-color:#123456');
   });
+
+  test('a slide with a data:image/svg+xml imageSrc exports a non-empty background-image url (safeImageUrl, not safeUrl)', () => {
+    const svgDataUri = 'data:image/svg+xml,%3Csvg%2F%3E';
+    const slidesWithSvg = [
+      { type: 'image' as const, imageSrc: svgDataUri, heading: 'One' },
+    ];
+    const { html } = toHtml({ slides: slidesWithSvg }, '');
+    expect(html).toContain(`background-image:url('${svgDataUri}')`);
+  });
 });
 
 describe('ContentSlider.toHtml interval is NOT runtime-type-checked -- must be coerced before it reaches the inline <script> numeric context', () => {