Init the attribute data-ag-theme-mode of ui.aggrid before themeObserver created (zauberzeug#5306)

### Motivation
If `ui.aggrid` was created in a page with dark mode, `ui.aggrid`'s theme
won't be dark mode unless resizing page.
<!-- What problem does this PR solve? Which new feature or improvement
does it implement? -->
<!-- Please provide relevant links to corresponding issues and feature
requests. -->

### Implementation

Add `this.$el.setAttribute("data-ag-theme-mode",
document.body.classList.contains("body--dark") ? "dark" : "light")` to
aggrid.js line 8.
Before themeObserver created, init the attribute `data-ag-theme-mode` of
`ui.aggrid`.
<!-- What is the concept behind the implementation? How does it work?
-->
<!-- Include any important technical decisions or trade-offs made -->

### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] Pytests have been added (or are not necessary).
- [x] Documentation has been added (or is not necessary).

---------

Co-authored-by: Falko Schindler <[email protected]>

Author: python-and-novella

nicegui/elements/aggrid/aggrid.js

@@ -6,10 +6,11 @@ export default {
   mounted() {
     this.update_grid();
 
-    this.themeObserver = new MutationObserver(() =>
-      this.$el.setAttribute("data-ag-theme-mode", document.body.classList.contains("body--dark") ? "dark" : "light")
-    );
+    const updateTheme = () =>
+      this.$el.setAttribute("data-ag-theme-mode", document.body.classList.contains("body--dark") ? "dark" : "light");
+    this.themeObserver = new MutationObserver(updateTheme);
     this.themeObserver.observe(document.body, { attributes: true, attributeFilter: ["class"] });
+    updateTheme();
   },
   unmounted() {
     this.themeObserver.disconnect();