Make the release (R8) build pass

Enabling R8 in the previous commit surfaced two build issues; fixed here so
assembleDebug AND assembleRelease both succeed (verified on this machine:
Android Studio JBR 21 + SDK android-36).

- proguard-rules.pro: add `-dontwarn com.google.errorprone.annotations.**`.
  Tink (pulled in by androidx.security-crypto / EncryptedSharedPreferences)
  references compile-only Error Prone annotations that aren't on the runtime
  classpath, which R8 treated as fatal "missing class" errors.
- ApiClient.await(): @OptIn(ExperimentalCoroutinesApi::class) for the
  resume(value){ onCancellation } overload (experimental in coroutines 1.8.1).

Release APK is R8-shrunk (~1.8 MB vs ~18 MB debug). It is unsigned — wire a
signing config / keystore (kept out of VCS) before distributing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/proguard-rules.pro

@@ -33,3 +33,8 @@
 -dontwarn org.conscrypt.**
 -dontwarn org.bouncycastle.**
 -dontwarn org.openjsse.**
+
+# ─── Tink / EncryptedSharedPreferences (androidx.security-crypto) ─────────────
+# Tink references compile-only Error Prone annotations that aren't on the runtime
+# classpath; they're not needed at runtime, so silence the missing-class errors.
+-dontwarn com.google.errorprone.annotations.**

app/src/main/java/no/svorka/mcms/core/ApiClient.kt

@@ -1,6 +1,7 @@
 package no.svorka.mcms.core
 
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.suspendCancellableCoroutine
 import kotlinx.coroutines.withContext
 import kotlinx.serialization.json.JsonElement
@@ -24,6 +25,7 @@ import kotlin.coroutines.resumeWithException
 private fun Response.closeQuietly() { runCatching { close() } }
 
 /** Suspend bridge for an OkHttp call, with coroutine cancellation. */
+@OptIn(ExperimentalCoroutinesApi::class)   // resume(value) { onCancellation } overload
 suspend fun Call.await(): Response = suspendCancellableCoroutine { cont ->
     enqueue(object : Callback {
         override fun onResponse(call: Call, response: Response) {