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/api/handlers/management/handler.go

@@ -15,7 +15,6 @@ import (
 	"github.com/gin-gonic/gin"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/buildinfo"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
-	"github.com/router-for-me/CLIProxyAPI/v6/internal/usage"
 	sdkAuth "github.com/router-for-me/CLIProxyAPI/v6/sdk/auth"
 	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
 	"golang.org/x/crypto/bcrypt"
@@ -41,7 +40,6 @@ type Handler struct {
 	attemptsMu          sync.Mutex
 	failedAttempts      map[string]*attemptInfo // keyed by client IP
 	authManager         *coreauth.Manager
-	usageStats          *usage.RequestStatistics
 	tokenStore          coreauth.Store
 	localPassword       string
 	allowRemoteOverride bool
@@ -60,7 +58,6 @@ func NewHandler(cfg *config.Config, configFilePath string, manager *coreauth.Man
 		configFilePath:      configFilePath,
 		failedAttempts:      make(map[string]*attemptInfo),
 		authManager:         manager,
-		usageStats:          usage.GetRequestStatistics(),
 		tokenStore:          sdkAuth.GetTokenStore(),
 		allowRemoteOverride: envSecret != "",
 		envSecret:           envSecret,
@@ -124,9 +121,6 @@ func (h *Handler) SetAuthManager(manager *coreauth.Manager) {
 	h.mu.Unlock()
 }
 
-// SetUsageStatistics allows replacing the usage statistics reference.
-func (h *Handler) SetUsageStatistics(stats *usage.RequestStatistics) { h.usageStats = stats }
-
 // SetLocalPassword configures the runtime-local password accepted for localhost requests.
 func (h *Handler) SetLocalPassword(password string) { h.localPassword = password }