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/pubspec.yaml

@@ -30,10 +30,7 @@ dev_dependencies:
 
 flutter:
   uses-material-design: true
-  # assets:
-  #   - assets/svg/
-  #   - assets/error-copy.json
-  #   - shared/fabledsword.tokens.json
-  # NOTE: Task 5 will sync assets/ + shared/ and uncomment the block above.
-  # The flutter test runner refuses to build a bundle with missing asset
-  # paths, even though `flutter pub get` and `flutter analyze` only warn.
+  assets:
+    - assets/svg/
+    - assets/error-copy.json
+    - shared/fabledsword.tokens.json