Select active recording if nothing else is selected (#5627)

### What
If nothing is selected (or user presses escape), the active recording
gets selected.

In a future PR I will improve the contents of the selection panel when a
recording is selected, e.g. show the blueprints in the recording etc,
but for that I need to do some refactors first.

### Checklist
* [x] 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)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/5627/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5627/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/5627/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5627)
- [Docs
preview](https://rerun.io/preview/88a9b028b16bf7c2750b808ba4b10b49393e81a1/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/88a9b028b16bf7c2750b808ba4b10b49393e81a1/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_viewer/src/app_state.rs

@@ -296,12 +296,22 @@ impl AppState {
             ui,
             app_blueprint.time_panel_expanded,
         );
-        selection_panel.show_panel(
-            &ctx,
-            ui,
-            &mut viewport,
-            app_blueprint.selection_panel_expanded,
-        );
+
+        {
+            if ctx.selection().is_empty() {
+                // Make sure something is selected before showing the selection panel.
+                ctx.selection_state()
+                    .set_selection(re_viewer_context::Item::StoreId(
+                        ctx.entity_db.store_id().clone(),
+                    ));
+            }
+            selection_panel.show_panel(
+                &ctx,
+                ui,
+                &mut viewport,
+                app_blueprint.selection_panel_expanded,
+            );
+        }
 
         let central_panel_frame = egui::Frame {
             fill: ui.style().visuals.panel_fill,