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/assets/error-copy.json

@@ -0,0 +1,16 @@
+{
+  "unknown": "Something went wrong.",
+  "unauthenticated": "Your session has ended. Please sign in again.",
+  "connection_refused": "Couldn't reach the server. Check the URL and try again.",
+  "lidarr_unreachable": "Lidarr is unreachable right now. Try again, or check Admin → Integrations.",
+  "lidarr_disabled": "Lidarr integration is not enabled.",
+  "lidarr_auth_failed": "Lidarr authentication failed.",
+  "lidarr_defaults_incomplete": "Lidarr is missing a default quality profile or root folder. Set them in Admin → Integrations.",
+  "lidarr_server_error": "Lidarr returned an error. Check Lidarr's logs for the cause.",
+  "lidarr_rejected": "Lidarr rejected the request. Check the server logs for the field-level reason.",
+  "lidarr_album_lookup_failed": "Lidarr doesn't recognize this album. Try Resolve or Delete file instead.",
+  "album_mbid_missing": "This track has no Lidarr album to remove.",
+  "request_not_pending": "This request is no longer pending.",
+  "request_not_found": "That request no longer exists.",
+  "track_not_found": "That track no longer exists."
+}