feat(themes): bump JSON schema to v2 with typography roundtrip

Loader returns Theme? — null is the silent hard-cut skip for v1 files so
the v2.x refactor stays free of legacy-mapping code. v2 adds an optional
typography{} block with overrideGlobalFontSizePt and overrideSymbolsFontSizePt
slots, both nullable. ThemeRegistry.RefreshCustomCache gets a null guard
so the yield path drops skipped files cleanly. Writer emits typography{}
with explicit nulls so hand-edited files show the available knobs.

HellionChat/Themes/ThemeJsonLoader.cs

@@ -5,9 +5,13 @@ namespace HellionChat.Themes;
 
 internal static class ThemeJsonLoader
 {
-    public const int SupportedSchemaVersion = 1;
+    public const int SupportedSchemaVersion = 2;
 
-    public static Theme LoadFromString(string json)
+    // Returns null when the file declares an older schemaVersion. Hard-cut
+    // policy from the v2.x style refactor: v1 user themes are not migrated,
+    // they're silently ignored so the loader stays free of legacy mapping
+    // code. Any other malformed input still throws FormatException.
+    public static Theme? LoadFromString(string json)
     {
         if (string.IsNullOrWhiteSpace(json))
             throw new FormatException("Theme JSON is empty");
@@ -27,9 +31,11 @@ internal static class ThemeJsonLoader
             var root = doc.RootElement;
 
             var schemaVersion = ReadInt(root, "schemaVersion");
-            if (schemaVersion != SupportedSchemaVersion)
+            if (schemaVersion < SupportedSchemaVersion)
+                return null;
+            if (schemaVersion > SupportedSchemaVersion)
                 throw new FormatException(
-                    $"Unsupported schemaVersion {schemaVersion}; expected {SupportedSchemaVersion}"
+                    $"Unsupported schemaVersion {schemaVersion}; this build reads up to {SupportedSchemaVersion}"
                 );
 
             var slug = ReadString(root, "slug");
@@ -39,6 +45,7 @@ internal static class ThemeJsonLoader
 
             var colors = ReadColors(root.GetProperty("colors"));
             var layout = ReadLayout(root.GetProperty("layout"));
+            var typography = ReadTypography(root);
 
             ThemeChatColors? chatColors = null;
             if (
@@ -54,7 +61,7 @@ internal static class ThemeJsonLoader
                 description,
                 colors,
                 layout,
-                new ThemeTypography(),
+                typography,
                 IsBuiltIn: false,
                 ChatColors: chatColors
             );
@@ -86,7 +93,7 @@ internal static class ThemeJsonLoader
         return new ThemeChatColors(dict);
     }
 
-    public static Theme LoadFromFile(string path)
+    public static Theme? LoadFromFile(string path)
     {
         // FileShare.Read lets concurrent readers and well-behaved editors share
         // the handle; atomic-replace editors still raise IOException, caught upstream.
@@ -134,6 +141,20 @@ internal static class ThemeJsonLoader
             FrameBorderSize: ReadFloat(el, "frameBorderSize")
         );
 
+    // Optional in v2 — themes without a typography block default to the
+    // record's parameterless construction (both override slots null). A
+    // present-but-empty object also yields the default.
+    private static ThemeTypography ReadTypography(JsonElement root)
+    {
+        if (!root.TryGetProperty("typography", out var el) || el.ValueKind != JsonValueKind.Object)
+            return new ThemeTypography();
+
+        return new ThemeTypography(
+            OverrideGlobalFontSizePt: ReadOptionalFloat(el, "overrideGlobalFontSizePt"),
+            OverrideSymbolsFontSizePt: ReadOptionalFloat(el, "overrideSymbolsFontSizePt")
+        );
+    }
+
     private static string ReadString(JsonElement el, string name)
     {
         if (!el.TryGetProperty(name, out var v) || v.ValueKind != JsonValueKind.String)
@@ -154,4 +175,15 @@ internal static class ThemeJsonLoader
             throw new FormatException($"Theme JSON missing number property '{name}'");
         return (float)v.GetDouble();
     }
+
+    private static float? ReadOptionalFloat(JsonElement el, string name)
+    {
+        if (!el.TryGetProperty(name, out var v))
+            return null;
+        if (v.ValueKind == JsonValueKind.Null)
+            return null;
+        if (v.ValueKind != JsonValueKind.Number)
+            throw new FormatException($"Theme JSON property '{name}' must be a number or null");
+        return (float)v.GetDouble();
+    }
 }