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/data/update/AppUpdateChecker.kt

@@ -86,7 +86,7 @@ open class AppUpdateChecker(
                     .build()
             client.newCall(request).execute().use { response ->
                 if (!response.isSuccessful) return@use null
-                val body = response.body?.string().orEmpty()
+                val body = response.body.string().orEmpty()
                 parseUpdateInfo(body)
             }
         }
@@ -106,7 +106,7 @@ open class AppUpdateChecker(
             try {
                 client.newCall(request).execute().use { response ->
                     if (!response.isSuccessful) return@withContext false
-                    val body = response.body ?: return@withContext false
+                    val body = response.body
                     val total = body.contentLength()
                     dest.outputStream().use { out ->
                         val buffer = ByteArray(DEFAULT_BUFFER_SIZE)