fix(browser,cdp,js): about:blank navigation, real getBoxModel/getContentQuads, expose _wrap and self

- about: URL scheme now handled in navigate_single instead of
  hitting validate_url and failing with "Forbidden URL scheme"
- DOM.getBoxModel calls getBoundingClientRect via JS instead of
  returning hardcoded pixel coords
- DOM.getContentQuads added (was missing, needed by Playwright click)
- globalThis._wrap exposed so CDP eval code can look up nodes by id
- globalThis.self = globalThis for webpack/bundler worker detection

crates/obscura-browser/src/page.rs

@@ -611,6 +611,12 @@ impl Page {
             }
         }
 
+        if url.scheme() == "about" {
+            self.navigate_blank();
+            self.init_js();
+            return Ok(());
+        }
+
         let response = if url.scheme() == "data" {
             let content_type = url_str.strip_prefix("data:")
                 .and_then(|s| s.split(',').next())