chore: housekeeping — linter & formatter setup

Add .prettierrc.json, .markdownlint.json, .yamllint.yaml, .gitattributes
Run CSharpier, Prettier and markdownlint across the entire codebase.
No logic changes — formatting, using order and line endings only.

HellionChat/SelfTests/ThemeSwitchSelfTestStep.cs

@@ -40,7 +40,9 @@ internal sealed class ThemeSwitchSelfTestStep : ISelfTestStep
         if (this.initialSlug is null)
         {
             this.initialSlug = active.Slug;
-            ImGui.Text($"Initial theme: \"{this.initialSlug}\". Open Settings -> Theme & Layout and pick a different theme.");
+            ImGui.Text(
+                $"Initial theme: \"{this.initialSlug}\". Open Settings -> Theme & Layout and pick a different theme."
+            );
             return SelfTestStepResult.Waiting;
         }
 
@@ -77,9 +79,6 @@ internal sealed class ThemeSwitchSelfTestStep : ISelfTestStep
     private static bool HasPopulatedCache(Theme theme)
     {
         var cache = theme.AbgrCache;
-        return (cache.Primary
-              | cache.WindowBg
-              | cache.TextPrimary
-              | cache.Border) != 0u;
+        return (cache.Primary | cache.WindowBg | cache.TextPrimary | cache.Border) != 0u;
     }
 }