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/src/main/AndroidManifest.xml

@@ -68,5 +68,17 @@
             </intent-filter>
         </receiver>
 
+        <!-- Remove WorkManager's default initializer — app uses on-demand init via Configuration.Provider -->
+        <provider
+            android:name="androidx.startup.InitializationProvider"
+            android:authorities="${applicationId}.androidx-startup"
+            android:exported="false"
+            tools:node="merge">
+            <meta-data
+                android:name="androidx.work.WorkManagerInitializer"
+                android:value="androidx.startup"
+                tools:node="remove" />
+        </provider>
+
     </application>
 </manifest>