CI: serialize model_generator.sh + explicit Flutter precache step

The 'async' flag in model_generator.sh launches per-module flutter pub get
in parallel via 'bash -c ... &'. Flutter has a single-process startup lock
for SDK initialization — when N parallel calls race for it, most lose,
leaving the SDK cache half-initialized. Subsequent pub gets then fail
with the misleading 'doesn't support null safety' error because Flutter
can't resolve its own bundled sky_engine.

Two fixes:
1. Drop 'async' — model_generator.sh now runs the 8 modules sequentially.
   Adds maybe 3 minutes per CI run vs parallel, but reliable beats fast.
2. Add explicit 'flutter precache --linux' step before model_generator. The
   '|| true' absorbs the known flutter_gpu.zip 404 on Flutter 3.32.0 —
   that asset isn't actually published at the URL Flutter constructs and
   isn't needed for desktop Linux.

.github/workflows/build-linux.yml

@@ -150,8 +150,19 @@ jobs:
       - name: Generate per-module secrets.g.dart files
         run: dart run tool/generate_new_secrets.dart
 
+      # Initialize Flutter SDK once before parallel-ish work happens. flutter
+      # precache for Flutter 3.32.0 hits a 404 on flutter_gpu.zip (asset not
+      # actually published at the path Flutter constructs); the `|| true`
+      # absorbs that — desktop Linux build doesn't need flutter_gpu.
+      - name: Initialize Flutter SDK
+        run: |
+          flutter --version
+          flutter precache --linux --no-android --no-ios --no-macos --no-windows --no-fuchsia --no-web || true
+
+      # Sequential — async flag races on Flutter's startup lock and corrupts
+      # the SDK state, causing the "doesn't support null safety" cascade.
       - name: Build generated code (mobx + hive adapters)
-        run: bash model_generator.sh async
+        run: bash model_generator.sh
 
       - name: Generate localization
         run: dart run tool/generate_localization.dart