ci(flutter): switch to dedicated flutter-ci runner

The flutter-ci runner image (CI-Runner/CI-flutter/Dockerfile) bakes in
Flutter 3.41.8 + Android cmdline-tools + platform-34 + build-tools
34.0.0 + JDK 17, so the workflow no longer needs subosito/flutter-action
to install everything per-job. Drops the setup step and switches
runs-on from the general-purpose go-ci to flutter-ci.

Per project memory feedback_dedicated_runner_labels.md: prefer
purpose-built labels with toolchains pre-installed over generic-runner +
setup-action ceremony on this self-hosted Forgejo deployment.

.forgejo/workflows/flutter.yml

@@ -27,7 +27,10 @@ on:
 
 jobs:
   analyze-test-build:
-    runs-on: go-ci
+    # flutter-ci runner image (CI-Runner/CI-flutter/Dockerfile) bakes in
+    # Flutter SDK + Android cmdline-tools + platform-34 + build-tools 34.0.0
+    # + JDK 17 + a non-root `runner` user. No setup-action ceremony needed.
+    runs-on: flutter-ci
 
     defaults:
       run:
@@ -37,13 +40,6 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Setup Flutter
-        uses: subosito/flutter-action@v2
-        with:
-          flutter-version: '3.24.5'
-          channel: stable
-          cache: true
-
       - name: Sync shared assets from web/
         run: ./tool/sync_shared.sh