Fix R8 dontwarn rules, WorkManager init, and release signing config

- Add dontwarn for errorprone annotations (Tink) and sun.security.x509
- Remove WorkManagerInitializer from manifest (app uses on-demand init)
- Wire signingConfigs.release from local.properties

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

app/proguard-rules.pro

@@ -6,3 +6,5 @@
 -dontwarn org.bouncycastle.**
 -dontwarn org.conscrypt.**
 -dontwarn org.openjsse.**
+-dontwarn com.google.errorprone.annotations.**
+-dontwarn sun.security.x509.X509Key