feat(flutter): token + asset sync from web; gen_tokens.dart

shared/fabledsword.tokens.json mirrors web/src/lib/styles/tokens.json;
assets/error-copy.json mirrors the web's error-code copy table;
assets/svg/album-fallback.svg mirrors the web placeholder. Together
these give Flutter the same visual + copy starting point as the SPA.

tool/gen_tokens.dart emits lib/theme/tokens.dart from the JSON; the
generated file is committed so CI can verify the JSON and the Dart
file haven't drifted (gate added in Task 21).

tool/sync_shared.sh mkdirs the shared/ + assets/ destinations before
copying so the script works on a fresh checkout (the original
plan-text shape only mkdir'd assets/svg).

pubspec.yaml assets block uncommented now that the three asset paths
exist on disk.

flutter_client/shared/fabledsword.tokens.json

@@ -0,0 +1,34 @@
+{
+  "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"
+  },
+  "fontStacks": {
+    "display": "'Fraunces', Georgia, serif",
+    "body": "'Inter', system-ui, sans-serif",
+    "mono": "'JetBrains Mono', ui-monospace, monospace"
+  }
+}