[FIX] utils: Correct validation of mount target in shadow DOM/iframe

Previously, `validateTarget` only checked if the target element or its
host (if it was a ShadowRoot) was directly contained in the document body.
This failed in cases where the target element was nested inside a shadow
DOM, which itself was attached to the document.

This commit introduces a new helper `isAttachedToDocument` that
traverses through parent nodes and shadow roots to ensure that the
target is ultimately attached to the given document.
Additionally, it now throws a clear error if `document.defaultView` is
missing, indicating that the target document is detached or invalid.

This ensures proper validation of mount targets, including complex
scenarios with shadow roots and iframes.

tests/shadow_dom/__snapshots__/shadow_dom.test.ts.snap

@@ -27,6 +27,58 @@ exports[`shadow_dom can mount app 1`] = `
 }"
 `;
 
+exports[`shadow_dom can mount app in closed shadow dom 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`shadow_dom can mount app inside a separate HTML document 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`shadow_dom can mount app inside a shadow child element 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`shadow_dom can mount app inside an element in a shadow root inside an iframe 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div class=\\"my-div\\"/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
 exports[`shadow_dom useRef hook 1`] = `
 "function anonymous(app, bdom, helpers
 ) {