api: honor interception Continue overrides and fix pre-navigation enable

op_fetch_url ignored the url/method/headers/body fields on
InterceptResolution::Continue, so the interception channel could pass, mock, or
block a request but not rewrite it. Apply them: shadow url/method/body and
derive custom_headers from the override after the channel resolves Continue.

Also fix enable_interception() called before the first navigation: init_js
propagated intercept_tx to the new runtime but not intercept_enabled, so the
runtime started with interception disabled and op_fetch_url never intercepted.
Propagate intercept_enabled in init_js as well.

Add a test that rewrites a fetch's url via Continue and asserts the rewritten
response is captured.

crates/obscura-browser/src/page.rs

@@ -327,6 +327,11 @@ impl Page {
         if let Some(tx) = &self.intercept_tx {
             rt.set_intercept_tx(tx.clone());
         }
+        // Re-apply intercept_enabled: enable_interception()/enable_intercept()
+        // called before the first navigation sets this on the Page while the
+        // runtime does not exist yet, so the new runtime would otherwise start
+        // with interception disabled and op_fetch_url would never intercept.
+        rt.set_intercept_enabled(self.intercept_enabled);
 
         if let Some(dom) = self.dom.take() {
             rt.set_dom(dom);