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/shared/fabledsword.tokens.json

@@ -1,31 +1,36 @@
 {
   "colors": {
-    "obsidian": "#14171A",
-    "iron": "#1E2228",
-    "slate": "#2C313A",
-    "pewter": "#3F4651",
-    "parchment": "#E8E4D8",
-    "vellum": "#C2BFB4",
-    "ash": "#9C9A92",
-    "moss": "#4A5D3F",
-    "bronze": "#8B7355",
-    "oxblood": "#6B2118",
-    "warning": "#8B6F1E",
-    "error": "#C04A1F",
-    "info": "#3D5A6E",
-    "accent": "#4A6B5C"
-  },
-  "radii": {
-    "sm": "4px",
-    "md": "8px",
-    "lg": "12px",
-    "xl": "16px"
-  },
-  "fonts": {
-    "display": "Fraunces",
-    "body": "Inter",
-    "mono": "JetBrains Mono"
+    "dark": {
+      "obsidian": "#14171A",
+      "iron": "#1E2228",
+      "slate": "#2C313A",
+      "pewter": "#3F4651",
+      "parchment": "#E8E4D8",
+      "vellum": "#C2BFB4",
+      "ash": "#9C9A92"
+    },
+    "light": {
+      "obsidian": "#F8F5EE",
+      "iron": "#ECE6D5",
+      "slate": "#DCD3BD",
+      "pewter": "#B8AE94",
+      "parchment": "#14171A",
+      "vellum": "#2C313A",
+      "ash": "#5C6068"
+    },
+    "flat": {
+      "moss": "#4A5D3F",
+      "bronze": "#8B7355",
+      "oxblood": "#6B2118",
+      "warning": "#8B6F1E",
+      "error": "#C04A1F",
+      "info": "#3D5A6E",
+      "accent": "#4A6B5C",
+      "on-action": "#E8E4D8"
+    }
   },
+  "radii": { "sm": "4px", "md": "8px", "lg": "12px", "xl": "16px" },
+  "fonts": { "display": "Fraunces", "body": "Inter", "mono": "JetBrains Mono" },
   "fontStacks": {
     "display": "'Fraunces', Georgia, serif",
     "body": "'Inter', system-ui, sans-serif",