add cache for build

.github/workflows/android.yaml

@@ -14,6 +14,40 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v6
 
+      - name: Cache /nix/store (experimental, may be large)
+        uses: actions/cache@v4
+        with:
+          path: /nix/store
+          key:
+            ${{ runner.os }}-nix-store-${{ hashFiles('flake.lock','flake.nix')
+            }}
+          restore-keys: |
+            ${{ runner.os }}-nix-store-
+
+      - name: Cache Flutter Pub
+        uses: actions/cache@v4
+        with:
+          path: ~/.pub-cache
+          key:
+            ${{ runner.os }}-pub-${{ hashFiles('pubspec.lock','pubspec.yaml')
+            }}
+          restore-keys: |
+            ${{ runner.os }}-pub-
+
+      - name: Cache Gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+            android/.gradle
+          key:
+            ${{ runner.os }}-gradle-${{
+            hashFiles('android/gradle.properties','android/gradle/wrapper/gradle-wrapper.properties')
+            }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
       - name: Prepare Nix devshell
         uses: ./.github/actions/nix-devshell
         with:
@@ -26,6 +60,14 @@ jobs:
             cd lib/l10n
             python3 collect_translations.py
 
+      - name: Restore Dart packages
+        uses: ./.github/actions/nix-devshell
+        with:
+          commands: |
+            # ensure collected translations directory exists so pub get doesn't fail during code generation
+            mkdir -p lib/l10n/collected
+            flutter pub get
+
       - name: Build Debug APK
         uses: ./.github/actions/nix-devshell
         with: