chore: remove usage tracking and logging functionality

- Deleted the `LoggerPlugin` along with associated usage tracking and in-memory statistics logic.
- Removed all related tests (`logger_plugin_test.go`, `usage_tab_test.go`) and external-facing handler (`usage.go`) for usage statistics export/import.
- Cleaned up TUI integration by deleting `usage_tab.go`.

internal/redisqueue/usage_toggle.go

@@ -0,0 +1,16 @@
+package redisqueue
+
+import "sync/atomic"
+
+var usageStatisticsEnabled atomic.Bool
+
+func init() {
+	usageStatisticsEnabled.Store(true)
+}
+
+// SetUsageStatisticsEnabled toggles whether usage records are enqueued into the redisqueue payload buffer.
+// This is controlled by the config field `usage-statistics-enabled` and the corresponding management API.
+func SetUsageStatisticsEnabled(enabled bool) { usageStatisticsEnabled.Store(enabled) }
+
+// UsageStatisticsEnabled reports whether the usage queue plugin should publish records.
+func UsageStatisticsEnabled() bool { return usageStatisticsEnabled.Load() }