Show hidden url search param in app.rerun.io (#2455)

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### What

Fixes #2391

Upon loading `app.rerun.io` sets the `url` search param if it is not
already set

### Checklist
* [ ] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [ ] I've included a screenshot or gif (if applicable)

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PR Build Summary: https://build.rerun.io/pr/2455

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Docs preview: https://rerun.io/preview/a0fa005/docs
Examples preview: https://rerun.io/preview/a0fa005/examples
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web_viewer/index.html

@@ -367,7 +367,9 @@
             const rrd_file = url_params.get('file') || 'colmap_fiat.rrd';
 
             // Normalize the extra slashes from the url
-            return (data_path + rrd_file).replace(/\/{2,}/g, '/');
+            const normalized = (data_path + rrd_file).replace(/\/{2,}/g, '/');
+            window.location.search = `?url=${normalized}`;
+            return normalized;
         }
 
         function on_wasm_error(error) {