v1.6: fix HIGH security vuln — remove plaintext cookie backup

- Remove KEY_COOKIES_BACKUP plaintext fallback from PreferencesManager
- getCookies() now fails closed (force re-login) if EncryptedSharedPreferences unavailable
- Set android:allowBackup="false" to prevent adb backup extraction of session data
- Add missing gradle-wrapper.jar to repo

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

gradle/wrapper/gradle-wrapper.properties

@@ -1,7 +1,5 @@
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.6-bin.zip
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.6-bin.zip
-networkTimeout=10000
-validateDistributionUrl=true
-zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
+zipStoreBase=GRADLE_USER_HOME