ci: pin flutter to 3.44.1 in all workflows

Workflows used subosito/flutter-action with no version, silently tracking
GitHub's latest stable. When stable rolled to 3.44.1 it broke the build with
no code change. Pin explicitly so builds are reproducible and Flutter upgrades
are a deliberate bump rather than a surprise.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -13,6 +13,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: subosito/flutter-action@v2
         with:
+          flutter-version: 3.44.1
           cache: true
       - run: flutter doctor -v
       - run: flutter pub get

.github/workflows/gh-release.yml

@@ -32,6 +32,7 @@ jobs:
       - name: Set up Flutter
         uses: subosito/flutter-action@v2
         with:
+          flutter-version: 3.44.1
           cache: true
       
       # Decode and create the upload-keystore.jks file, used for signing the app.

.github/workflows/pr-check.yml

@@ -10,6 +10,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: subosito/flutter-action@v2
         with:
+          flutter-version: 3.44.1
           cache: true
       - run: flutter doctor -v
       - run: flutter pub get

.github/workflows/store-release.yml

@@ -49,6 +49,7 @@ jobs:
       - name: Set up Flutter
         uses: subosito/flutter-action@v2
         with:
+          flutter-version: 3.44.1
           cache: true
 
       # Decode and create the upload-keystore.jks file, used for signing the app.
@@ -100,6 +101,7 @@ jobs:
       - name: Set up Flutter
         uses: subosito/flutter-action@v2
         with:
+          flutter-version: 3.44.1
           cache: true
 
       # Setup the iOS project with environment variables without signing (building and signing will happen in fastlane).