Add consistent APK signing so updates install without uninstalling

- Generate pixel10.keystore and store as KEYSTORE_BASE64 GitHub secret
- Release workflow decodes keystore and signs every APK with same key
- build.gradle.kts: signingConfig reads from env (CI) or local keystore
- Bump versionCode 8 / versionName 1.8.0

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

.github/workflows/release.yml

@@ -30,7 +30,16 @@ jobs:
           key: gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
           restore-keys: gradle-
 
-      - name: Build debug APK
+      - name: Decode signing keystore
+        run: |
+          echo "${{ secrets.KEYSTORE_BASE64 }}" | base64 -d > "$RUNNER_TEMP/pixel10.keystore"
+
+      - name: Build signed debug APK
+        env:
+          KEYSTORE_PATH: ${{ runner.temp }}/pixel10.keystore
+          KEYSTORE_PASSWORD: ${{ secrets.KEYSTORE_PASSWORD }}
+          KEY_ALIAS: ${{ secrets.KEY_ALIAS }}
+          KEY_PASSWORD: ${{ secrets.KEY_PASSWORD }}
         run: ./gradlew assembleDebug --no-daemon
 
       - name: Rename APK with version tag