fix(#284): remove dead code across API, models, WS client, and docs (#299)

Remove six dead-code findings from the automated review:
- CODE-01: updateProfileDescription endpoint + UpdateProfileDescriptionRequest DTO (uncalled)
- CODE-02: getConfig() endpoint (app uses getRawConfig())
- CODE-03: TogglePluginRequest DTO (unreferenced)
- CODE-05: mark isConnected with @VisibleForTesting (test-only usage)
- CODE-06: fix KDoc createdAt -> timestamp to match actual field name

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

app/src/main/java/com/m57/hermescontrol/data/ws/HermesWsClient.kt

@@ -1,6 +1,7 @@
 package com.m57.hermescontrol.data.ws
 
 import android.util.Log
+import androidx.annotation.VisibleForTesting
 import com.google.gson.Gson
 import com.m57.hermescontrol.BuildConfig
 import com.m57.hermescontrol.data.local.AuthManager
@@ -95,6 +96,7 @@ object HermesWsClient {
 
     // ── Connection helpers ────────────────────────────────────────────────
 
+    @VisibleForTesting
     val isConnected: Boolean get() = connected.get()
 
     /** Open a WebSocket connection using settings from [AuthManager]. */