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/tool/gen_tokens.dart

@@ -1,15 +1,25 @@
 // Reads shared/fabledsword.tokens.json and emits lib/theme/tokens.dart.
 // Run via `dart run tool/gen_tokens.dart`. The generated file is
 // committed (CI validates it matches the JSON to catch drift).
+//
+// Web tokens.json splits colors into `dark`, `light`, and `flat` blocks
+// for the SPA's theme toggle. Flutter v1 ships dark-only, so we merge
+// dark + flat into a single flat namespace. Light variants will be
+// emitted alongside as part of the Flutter theme-toggle parity work.
 import 'dart:convert';
 import 'dart:io';
 
 void main() {
   final jsonFile = File('shared/fabledsword.tokens.json');
   final tokens = jsonDecode(jsonFile.readAsStringSync()) as Map<String, dynamic>;
-  final colors = (tokens['colors'] as Map).cast<String, String>();
-  final radii  = (tokens['radii']  as Map).cast<String, String>();
-  final fonts  = (tokens['fonts']  as Map).cast<String, String>();
+
+  final colorsRoot = (tokens['colors'] as Map).cast<String, dynamic>();
+  final dark = (colorsRoot['dark'] as Map).cast<String, String>();
+  final flat = (colorsRoot['flat'] as Map).cast<String, String>();
+  final colors = <String, String>{...dark, ...flat};
+
+  final radii = (tokens['radii'] as Map).cast<String, String>();
+  final fonts = (tokens['fonts'] as Map).cast<String, String>();
 
   final out = StringBuffer()
     ..writeln('// GENERATED — do not edit. Source: shared/fabledsword.tokens.json')
@@ -38,5 +48,11 @@ void main() {
   stdout.writeln('wrote lib/theme/tokens.dart');
 }
 
-String _camel(String s) => s; // tokens are already lowerCamel-friendly
-String _pascal(String s)  => s[0].toUpperCase() + s.substring(1);
+// Hyphens become camelCase boundaries: on-action → onAction.
+String _camel(String s) {
+  final parts = s.split('-');
+  return parts.first +
+      parts.skip(1).map((p) => p[0].toUpperCase() + p.substring(1)).join();
+}
+
+String _pascal(String s) => s[0].toUpperCase() + s.substring(1);