feat(themes): theme registry with built-in lookup and fallback

HellionChat/Themes/ThemeRegistry.cs

@@ -0,0 +1,42 @@
+using HellionChat.Themes.Builtin;
+
+namespace HellionChat.Themes;
+
+public sealed class ThemeRegistry
+{
+    public const string DefaultSlug = HellionArctic.Slug;
+
+    private readonly Dictionary<string, Theme> _builtIns;
+    private Theme _active;
+
+    public ThemeRegistry()
+    {
+        _builtIns = new Dictionary<string, Theme>(StringComparer.OrdinalIgnoreCase)
+        {
+            { HellionArctic.Slug, HellionArctic.Build() },
+            { Chat2Classic.Slug,  Chat2Classic.Build() },
+            { EventHorizon.Slug,  EventHorizon.Build() },
+            { MoonlitBloom.Slug,  MoonlitBloom.Build() },
+            { MintGrove.Slug,     MintGrove.Build() },
+        };
+        _active = _builtIns[DefaultSlug];
+    }
+
+    public Theme Active => _active;
+
+    // Active-Lookup: liefert das angeforderte Theme oder fällt auf Hellion Arctic
+    // zurück, wenn der Slug unbekannt ist.
+    public Theme Get(string slug)
+    {
+        return _builtIns.TryGetValue(slug, out var theme)
+            ? theme
+            : _builtIns[DefaultSlug];
+    }
+
+    public IEnumerable<Theme> AllBuiltIns() => _builtIns.Values;
+
+    public void Switch(string slug)
+    {
+        _active = Get(slug);
+    }
+}