security: add safeImageUrl, un-break M-5's over-blocking of image-context SVG data URIs

M-5 made safeUrl() block data:image/svg+xml everywhere, including the
image-only sinks (<img src>, CSS url()) that Gallery's default images and
other SVG placeholders rely on. Loaded as an image, an SVG is rasterized
and never executes an inline <script>/onload= -- that only happens when
it's navigated to or loaded as an <iframe> document -- so M-5 over-blocked
the safe contexts and broke every published Gallery (and other components
using an SVG placeholder) using safeUrl's default images in prod.

Adds safeImageUrl(): identical javascript:/vbscript: handling to safeUrl,
but treats data: as an allowlist of image/* subtypes instead of a
blocklist -- allows all data:image/* (including svg+xml, with or without
base64), still blocks data:text/html and any other non-image data: type.

Swapped to safeImageUrl at IMAGE-src / CSS-image url() sinks only:
- Gallery.tsx img src + lightbox data-lb-src
- ImageBlock.tsx img src (toHtml)
- Logo.tsx / Navbar.tsx logo <img> src (their href/link targets keep safeUrl)
- style-helpers.ts sanitizeCssValue's url(...) handling (background-image
  for HeroSimple/BackgroundSection/Section/CallToAction)

Left on safeUrl (href/iframe/form-action/navigation sinks, where
data:image/svg+xml must stay blocked): ButtonLink, Icon link, SocialLinks,
Menu/Navbar link hrefs, PricingTable buttonHref, _cta-helpers,
ContentSlider buttonHref, FeaturesGrid buttonUrl, FormContainer action
(via form-relay-wiring), MapEmbed/VideoBlock iframe src.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/sections/Gallery.toHtml.test.ts

@@ -94,6 +94,32 @@ describe('Gallery.toHtml deterministic + unique scope ids (thread node id, no Ma
   });
 });
 
+describe('Gallery.toHtml default SVG placeholder images survive export (Bug 2 regression)', () => {
+  test('a default data:image/svg+xml image emits a non-empty img src, not src=""', () => {
+    const { html } = toHtml({}, ''); // no images prop -> component default SVG placeholders
+    expect(html).not.toContain('src=""');
+    expect(html).toMatch(/src="data:image\/svg\+xml[^"]*"/);
+  });
+
+  test('an explicit data:image/svg+xml gallery image src is preserved (not stripped to empty)', () => {
+    const svg = 'data:image/svg+xml,%3Csvg%2F%3E';
+    const { html } = toHtml({ images: [{ src: svg, alt: 'a' }] }, '');
+    expect(html).toContain(`src="${svg}"`);
+  });
+
+  test('lightbox data-lb-src also preserves data:image/svg+xml (still an image context)', () => {
+    const svg = 'data:image/svg+xml,%3Csvg%2F%3E';
+    const { html } = toHtml({ images: [{ src: svg, alt: 'a' }], lightbox: true }, '');
+    expect(html).toContain(`data-lb-src="${svg}"`);
+  });
+
+  test('a javascript: gallery image src still yields an empty src (safeImageUrl still blocks it)', () => {
+    const { html } = toHtml({ images: [{ src: 'javascript:alert(1)', alt: 'a' }] }, '');
+    expect(html).toContain('src=""');
+    expect(html).not.toContain('javascript:');
+  });
+});
+
 describe('Gallery.toHtml lightbox focus management (M-2)', () => {
   const props = { images: [{ src: '/a.jpg', alt: 'a' }], lightbox: true };