fix(flutter): handle nested colors split in tokens.json codegen

Web tokens.json was reshaped for the SPA theme toggle (#362) with
colors split into dark/light/flat blocks. gen_tokens.dart still
expected a flat string map and threw _Map<String,dynamic> cast
errors in the flutter CI workflow. Read colors.dark + colors.flat
and merge into the existing flat namespace; light variants will be
emitted when the Flutter client gains a theme toggle as part of
#356 parity work. Adds hyphen→camelCase normalization so the new
on-action token surfaces as onAction.

flutter_client/lib/theme/tokens.dart

@@ -17,6 +17,7 @@ class FabledSwordTokens {
   static const Color error = Color(0xFFC04A1F);
   static const Color info = Color(0xFF3D5A6E);
   static const Color accent = Color(0xFF4A6B5C);
+  static const Color onAction = Color(0xFFE8E4D8);
   static const double radiusSm = 4;
   static const double radiusMd = 8;
   static const double radiusLg = 12;