fix(font): fall back to system font if embedded resource missing (F10.2)

GetHellionFontBytes used to throw a FileNotFoundException when the
embedded Hellion font resource was missing — only possible on a broken
csproj or a hand-rolled dev build, never on a signed release, but the
throw bubbled up and broke the entire UiBuilder font atlas.

Replaced with a nullable TryGetHellionFontBytes that logs a warning
and returns null on miss. The RegularFont delegate now falls back to
the same system-font path that UseHellionFont=false already uses, so
the plugin still loads and the issue surfaces in /xllog instead of as
a crash.

HellionChat/FontManager.cs

@@ -44,16 +44,26 @@ public class FontManager
     // Hellion font bytes (Exo 2, OFL-1.1); lazily loaded from manifest resources
     private static byte[]? HellionFontBytes;
 
-    private static byte[] GetHellionFontBytes()
+    // Returns null when the embedded font resource is missing. Should never
+    // happen on a signed release build, but a broken csproj or hand-rolled
+    // dev build can land here. Caller falls back to the system font path so
+    // the plugin still loads instead of crashing the whole UiBuilder.
+    private static byte[]? TryGetHellionFontBytes()
     {
         if (HellionFontBytes is not null)
             return HellionFontBytes;
 
-        using var stream =
-            typeof(FontManager).Assembly.GetManifestResourceStream("HellionFont.ttf")
-            ?? throw new FileNotFoundException(
-                "Hellion font resource not embedded in the assembly"
+        using var stream = typeof(FontManager).Assembly.GetManifestResourceStream(
+            "HellionFont.ttf"
+        );
+        if (stream is null)
+        {
+            Plugin.Log.Warning(
+                "Hellion font resource missing — falling back to system default font."
             );
+            return null;
+        }
+
         using var ms = new MemoryStream();
         stream.CopyTo(ms);
         HellionFontBytes = ms.ToArray();
@@ -146,8 +156,11 @@ public class FontManager
                     ? Plugin.Config.FontSizeV2
                     : Plugin.Config.GlobalFontV2.SizePt;
                 var config = new SafeFontConfig { SizePt = basePt, GlyphRanges = Ranges };
-                config.MergeFont = Plugin.Config.UseHellionFont
-                    ? tk.AddFontFromMemory(GetHellionFontBytes(), config, "Hellion-Exo2")
+                // F10.2: if the embedded font is missing, drop to the system font
+                // path rather than letting the UiBuilder throw.
+                var hellionBytes = Plugin.Config.UseHellionFont ? TryGetHellionFontBytes() : null;
+                config.MergeFont = hellionBytes is not null
+                    ? tk.AddFontFromMemory(hellionBytes, config, "Hellion-Exo2")
                     : AddFontWithFallback(tk, Plugin.Config.GlobalFontV2.FontId, config, "global");
 
                 config.SizePt = Plugin.Config.JapaneseFontV2.SizePt;