fix(android): use explicit serializer form for encodeToString

Adding `<ResumePayload>` to encodeToString made it bind to the wrong
overload — the compiler picked `encodeToString(SerializationStrategy<T>,
T)` and tried to treat `payload` as the serializer. Switched to the
explicit form:

  json.encodeToString(ResumePayload.serializer(), payload)

Always unambiguous. (Decode is fine — `decodeFromString<T>(String)`
isn't overloaded the same way.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/player/ResumeController.kt

@@ -88,7 +88,9 @@ class ResumeController @Inject constructor(
             positionMs = state.positionMs,
         )
         dao.upsert(
-            CachedResumeStateEntity(json = json.encodeToString<ResumePayload>(payload)),
+            CachedResumeStateEntity(
+                json = json.encodeToString(ResumePayload.serializer(), payload),
+            ),
         )
     }
 }