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/tool/sync_shared.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Copies shared assets from web/ into flutter_client/. Run before
+# `flutter build` and as part of CI. Idempotent.
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+mkdir -p shared assets/svg
+cp ../web/src/lib/styles/tokens.json     shared/fabledsword.tokens.json
+cp ../web/src/lib/styles/error-copy.json assets/error-copy.json
+cp ../web/static/placeholders/album-fallback.svg assets/svg/album-fallback.svg
+
+echo "shared assets synced from web/"