fix(builder): preserve data-URI semicolons in css value sanitizer

sanitizeCssValue's blanket `;` strip ran on the whole value AFTER url(...)
content was already safely re-wrapped, corrupting legitimate
data:image/png;base64,... URLs pasted into Background Image fields
(the MIME/base64 separator `;` was deleted, breaking the data URI in
exported/published HTML). Scope the `;`/`"` breakout sanitization to the
segments outside url(...) matches only -- the url() branch is already
fully safe via escapeAttr(safeUrl(...)) and must not be re-stripped.

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

craft/src/utils/style-helpers.ts

@@ -6,24 +6,41 @@ const camelToKebab = (str: string): string =>
 
 const URL_RE = /url\(\s*(['"]?)([\s\S]*?)\1\s*\)/gi;
 
+// Outside of a url(...) reference, a `;` is never legitimate (declarations
+// are separated by it) -- stray semicolons are how a breakout injects a
+// second property -- and a raw `"` would close the `style="..."` attribute
+// early. Inside url('...') the content has already been made safe via
+// escapeAttr(safeUrl(...)), including any `;` required by data-URI syntax
+// (`data:<mime>;base64,<payload>`), so this must never be applied there.
+const sanitizeBreakoutChars = (s: string): string => s.replace(/;/g, '').replace(/"/g, '&quot;');
+
 /**
  * Sanitizes a single CSS declaration value so it can never terminate the
  * `style="..."` attribute early, inject an extra declaration via a stray
  * `;`, or smuggle a `javascript:`/`vbscript:`/`data:text/html` URL through
  * a `url(...)` reference. Legitimate multi-part values (box-shadow,
  * gradients, etc.) that contain none of these characters pass through
- * unchanged.
+ * unchanged, and legitimate `;`-containing data-URIs inside url(...) are
+ * preserved intact.
  */
 function sanitizeCssValue(raw: string): string {
-  // Neutralize url(...) references: validate/strip the scheme and re-wrap
-  // in single quotes with the contents escaped for attribute safety.
-  let val = raw.replace(URL_RE, (_m, _q, inner) => `url('${escapeAttr(safeUrl(inner.trim()))}')`);
-  // A `;` in a CSS value is never legitimate (declarations are separated by
-  // it) -- stray semicolons are how a breakout injects a second property.
-  val = val.replace(/;/g, '');
-  // Any remaining raw `"` would close the `style="..."` attribute early.
-  val = val.replace(/"/g, '&quot;');
-  return val;
+  let out = '';
+  let lastIndex = 0;
+  URL_RE.lastIndex = 0;
+  let m: RegExpExecArray | null;
+  while ((m = URL_RE.exec(raw)) !== null) {
+    // Sanitize breakout characters only in the segment before this url(...)
+    // reference -- never inside the reference itself.
+    out += sanitizeBreakoutChars(raw.slice(lastIndex, m.index));
+    // Neutralize the url(...) reference: validate/strip the scheme and
+    // re-wrap in single quotes with the contents escaped for attribute
+    // safety. This is already fully safe, `;` and all.
+    const inner = m[2];
+    out += `url('${escapeAttr(safeUrl(inner.trim()))}')`;
+    lastIndex = URL_RE.lastIndex;
+  }
+  out += sanitizeBreakoutChars(raw.slice(lastIndex));
+  return out;
 }
 
 export function cssPropsToString(style: CSSProperties | undefined): string {