Apply IGMH brand colours, shrink subheadings, show page title as h1

- app.css: adopt the school's own colours (IGMH blue for header/links/headings, IGMH green as accent), light/dark; cooler near-neutral page bg
- app.css: reduce h2/h3 and give h4 its own size for a calmer heading scale
- ContentPage: render the page title as a visible <h1>, not only <PageTitle>

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Elternbeirat.Web/Features/Pages/ContentPage.razor

@@ -8,6 +8,7 @@
 else if (_page is not null)
 {
     <PageTitle>@_page.Title</PageTitle>
+    <h1>@_page.Title</h1>
     <article>@Body</article>
     <PageEmbeds Embed="_page.Embed" />
 }