github: Fix weblate action's rights, and update generated code.

.github/workflows/update-translations.yml

@@ -1,22 +1,56 @@
 name: Update translations from Weblate
+permissions:
+  contents: write
+  pull-requests: write
 on:
+  pull_request:
   schedule:
     - cron: "0 10 * * 1"
   workflow_dispatch:
+
 jobs:
   update-translations:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
       - name: Fetch and merge from Weblate
         # The commit message is generated in Weblate; see https://hosted.weblate.org/addon/17163/
         run: |
+          git checkout -b main
+          git log --oneline --graph --all -n5
           git remote add weblate https://hosted.weblate.org/git/zulip/zulip-flutter/
           git fetch weblate
-          git merge --ff-only weblate/main
+          GIT_COMMITTER_NAME="Hosted Weblate" GIT_COMMITTER_EMAIL="[email protected]" \
+            git cherry-pick weblate/main ^HEAD
+
+      - name: Clone Flutter SDK
+        # TODO(#1204) reinstate shallow clone with --depth=1000 and its corresponding comment here
+        run: |
+          git clone -b main https://github.com/flutter/flutter ~/flutter
+          TZ=UTC git --git-dir ~/flutter/.git log -1 --format='%h | %ci | %s' --date=iso8601-local
+          echo ~/flutter/bin >> "$GITHUB_PATH"
+
+          # The Flutter tool assumes the tip of tree is "origin/master"
+          # (or "upstream/master"):
+          #   https://github.com/flutter/flutter/issues/160626
+          # TODO(upstream): make workaround unneeded
+          git --git-dir ~/flutter/.git update-ref refs/remotes/origin/master origin/main
+
+      - name: Update generated code
+        run: |
+          mkdir -p build
+          tools/check l10n --fix
+          git add lib/generated/l10n/
+          GIT_COMMITTER_NAME="Hosted Weblate" GIT_COMMITTER_EMAIL="[email protected]" \
+            git commit --amend -C HEAD
+
       - name: Create Pull Request
         uses: peter-evans/create-pull-request@v7
         with:
           branch: update-translations/weblate
           delete-branch: true
           title: Update translations from Weblate
+          base: ${{ github.head_ref }}