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/tui/client.go

@@ -140,11 +140,6 @@ func (c *Client) PutConfigYAML(yamlContent string) error {
 	return err
 }
 
-// GetUsage fetches usage statistics.
-func (c *Client) GetUsage() (map[string]any, error) {
-	return c.getJSON("/v0/management/usage")
-}
-
 // GetAuthFiles lists auth credential files.
 // API returns {"files": [...]}.
 func (c *Client) GetAuthFiles() ([]map[string]any, error) {