ci(release): use GitHub App token in release-prepare for PR creation

GITHUB_TOKEN is not permitted to create PRs in this repo. Switch to the
GitHub App token (same pattern as release-tag and release-publish) so
melos-action can successfully open the release PR.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: grdsdev

.github/workflows/release-prepare.yml

@@ -23,11 +23,18 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 15
     steps:
+      - name: Generate token
+        id: app-token
+        uses: actions/create-github-app-token@v2
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          private-key: ${{ secrets.PRIVATE_KEY }}
+
       - name: Checkout
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
-          token: ${{ secrets.GITHUB_TOKEN }}
+          token: ${{ steps.app-token.outputs.token }}
 
       - name: Setup Flutter
         uses: subosito/flutter-action@v2