Add GitHub Actions workflow for building and testing the app

.github/actions/yarn-install/action.yml

@@ -0,0 +1,35 @@
+name: Yarn Install
+
+description: Install Yarn v4 and run yarn install.
+
+inputs:
+  working_directory:
+    description: The directory to install dependencies in.
+    required: false
+    default: "."
+
+runs:
+  using: "composite"
+  steps:
+    - name: Setup Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version: 20
+    - name: Install Yarn v4
+      shell: bash
+      run: |
+        corepack enable
+        corepack prepare yarn@4.5.0 --activate
+    - name: Cache Yarn dependencies
+      uses: actions/cache@v4
+      with:
+        path: |
+          ${{ inputs.working_directory }}/node_modules
+          ~/.cache/yarn
+        key: ${{ runner.os }}-yarn-${{ hashFiles('${{ inputs.working_directory }}/yarn.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-yarn-
+    - name: Install dependencies
+      shell: bash
+      run: yarn install --immutable
+      working-directory: ${{ inputs.working_directory }}

.github/workflows/app.yml

@@ -0,0 +1,40 @@
+name: App CI
+
+on:
+  push:
+    paths:
+      - "app/**"
+      - ".github/workflows/app.yml"
+      - ".github/actions/**"
+
+jobs:
+  install:
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Dependencies
+        uses: ./.github/actions/yarn-install
+        with:
+          working_directory: ./app
+
+  lint:
+    needs: install
+    runs-on: ubuntu-latest
+    steps:
+      - name: Run linter
+        run: yarn lint
+        working-directory: ./app
+      - name: Run prettier
+        run: yarn fmt
+        working-directory: ./app
+
+  test:
+    needs: install
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install app dependencies
+        run: yarn install-app
+        working-directory: ./app
+      - name: Test
+        run: yarn test
+        working-directory: ./app