fix(net,js): route op_fetch_url + ES module loader through proxy_url (#139) (#151)

Fixes the remaining two paths from #139: JS-level fetch()/XHR
(op_fetch_url) and dynamic ES module imports (ObscuraModuleLoader) both
built their reqwest clients with `Client::builder().build()` — no
proxy — so every JS-initiated network call silently bypassed
BrowserContext.proxy_url. (The third path, StealthHttpClient, was
already fixed in main: page.rs:61 uses with_proxy.)

Changes:

* obscura-net: expose ObscuraHttpClient::proxy_url() so callers outside
  the crate can route their own reqwest requests through the same
  upstream proxy. The field was already there — just lacked an accessor.

* obscura-js/module_loader: add `proxy_url: Option<String>` and
  `with_proxy(base_url, proxy_url)`. ObscuraModuleLoader::new keeps the
  pre-#139 direct-connection behaviour. load() builds a reqwest client
  per import, threading the proxy and bubbling up "Invalid module proxy"
  on bad URLs.

* obscura-js/ops.rs: replace the process-wide
  `static SHARED_HTTP_CLIENT: OnceLock<reqwest::Client>` (which could
  never be reconfigured after first call) with a per-request
  `build_request_client(proxy_url)` that pulls the configured proxy out
  of SharedState.http_client.proxy_url() each call. Per-request build
  cost is ~1ms vs the network round-trip, and we no longer have a
  global singleton that lies about its config.

* obscura-js/runtime: add ObscuraJsRuntime::with_base_url_and_proxy.
  with_base_url delegates to it with None so existing tests / callers
  are unaffected.

* obscura-browser/page.rs: thread context.proxy_url through to
  with_base_url_and_proxy when constructing the JS runtime.

crates/obscura-browser/src/page.rs

@@ -136,7 +136,14 @@ impl Page {
             return;
         }
 
-        let mut rt = ObscuraJsRuntime::with_base_url(&self.url_string());
+        // Thread the BrowserContext's proxy through to the ES-module loader
+        // and op_fetch_url so dynamic imports and JS fetch() honour the
+        // configured upstream proxy (#139). When proxy_url is None this is
+        // equivalent to with_base_url() (direct connection).
+        let mut rt = ObscuraJsRuntime::with_base_url_and_proxy(
+            &self.url_string(),
+            self.context.proxy_url.clone(),
+        );
         rt.set_url(&self.url_string());
         rt.set_title(&self.title);