chore: eliminate all Kotlin compiler warnings in main source (#877)

* chore: eliminate all Kotlin compiler warnings in main source

* test(chat): fix cross-class coroutine leaks poisoning the unit suite

- ChatUpdateCommandTest: inject FakeSlashUsageStore — the real DataStore
  store inits on real IO threads with a relaxed-mock app (cacheDir NPE)
  and leaks an uncaught exception into the next runTest class.
- ChatUpdateCommandTest + SlashCommandDispatchRpcTest: mock
  ProfileSwitchCoordinator.switched (ChatViewModelTest pattern) — VMs
  otherwise park collectors on the singleton flow that crash later
  classes (ProfileSwitchCoordinatorTest) on stale-Main resumption.

Full suite: 944 tests, 0 failures (was 2 flaky per run).

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/billing/BillingScreen.kt

@@ -135,7 +135,7 @@ private fun BillingContent(
                     shape = RoundedCornerShape(12.dp),
                 ) {
                     Text(
-                        text = state.errorMessage ?: "",
+                        text = state.errorMessage,
                         modifier = Modifier.padding(12.dp),
                         style = MaterialTheme.typography.bodyMedium,
                         color = MaterialTheme.colorScheme.onErrorContainer,
@@ -152,7 +152,7 @@ private fun BillingContent(
                     shape = RoundedCornerShape(12.dp),
                 ) {
                     Text(
-                        text = state.actionMessage ?: "",
+                        text = state.actionMessage,
                         modifier = Modifier.padding(12.dp),
                         style = MaterialTheme.typography.bodyMedium,
                     )
@@ -207,7 +207,7 @@ private fun PlanCard(subscription: SubscriptionCurrent) {
             }
             if (subscription.pending_downgrade_display != null) {
                 Text(
-                    text = subscription.pending_downgrade_display ?: "",
+                    text = subscription.pending_downgrade_display,
                     style = MaterialTheme.typography.bodySmall,
                     color = MaterialTheme.colorScheme.onSurfaceVariant,
                     modifier = Modifier.padding(top = 4.dp),