Restructure into feature folders with code-behind and scoped CSS

Elternbeirat.Web/Shared/FrontMatter.cs

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+using Markdig;
+using Markdig.Extensions.Yaml;
+using Markdig.Syntax;
+
+namespace Elternbeirat.Web.Services;
+
+/// <summary>
+/// Reads simple <c>key: value</c> pairs from a Markdown file's YAML front
+/// matter. Kept deliberately minimal (one value per line, no nesting) — this is
+/// enough for titles and dates. YamlDotNet will take over once we need
+/// structured metadata. Front-matter keys stay German because the Markdown
+/// files are edited by German-speaking editors.
+/// </summary>
+public static class FrontMatter
+{
+    /// <summary>
+    /// Returns the value of the given front-matter key, or <c>null</c> if the
+    /// file has no front matter or the key is missing. The key match is
+    /// case-insensitive; surrounding quotes on the value are removed.
+    /// </summary>
+    public static string? Read(string key, MarkdownDocument document, string source)
+    {
+        var block = document.Descendants<YamlFrontMatterBlock>().FirstOrDefault();
+        if (block is null)
+        {
+            return null;
+        }
+
+        var prefix = key + ":";
+        var frontMatter = source.Substring(block.Span.Start, block.Span.Length);
+        foreach (var line in frontMatter.Split('\n'))
+        {
+            var trimmed = line.Trim();
+            if (trimmed.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+            {
+                return trimmed[prefix.Length..].Trim().Trim('"');
+            }
+        }
+
+        return null;
+    }
+}