Cache Flutter SDK, Gradle, and CocoaPods in CI

Cold Gradle builds were taking ~8.5 min for the APK job. Add
caching for Flutter/pub (via flutter-action), Gradle deps, and
CocoaPods to speed up subsequent runs.

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

.github/workflows/pr.yml

@@ -16,6 +16,7 @@ jobs:
       - uses: subosito/flutter-action@v2
         with:
           channel: 'stable'
+          cache: true
 
       - run: flutter pub get
 
@@ -40,6 +41,15 @@ jobs:
       - uses: subosito/flutter-action@v2
         with:
           channel: 'stable'
+          cache: true
+
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: gradle-${{ runner.os }}-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: gradle-${{ runner.os }}-
 
       - run: flutter pub get
 
@@ -69,6 +79,13 @@ jobs:
       - uses: subosito/flutter-action@v2
         with:
           channel: 'stable'
+          cache: true
+
+      - uses: actions/cache@v4
+        with:
+          path: ios/Pods
+          key: pods-${{ runner.os }}-${{ hashFiles('ios/Podfile.lock') }}
+          restore-keys: pods-${{ runner.os }}-
 
       - run: flutter pub get