feat(online): use xray online-stats API for onlines and access-log-free IP limit

Adopt xray-core's statsUserOnline policy and GetUsersStats RPC so online
detection is connection-based and IP limiting no longer requires an access
log. Falls back to the legacy traffic-delta onlines and access-log parsing
when the running core lacks the RPCs (Unimplemented), probed lazily per
process so a panel-driven version switch re-evaluates automatically.

Backend:
- xray/api.go: GetOnlineUsers (one GetUsersStats call returns all online
  users and their source IPs) and IsUnimplementedErr.
- xray/process.go: per-process OnlineAPISupport tri-state capability cache.
- service/xray.go: ensureStatsPolicy injects statsUserOnline into every
  policy level of the generated config; XrayService.GetOnlineUsers probes
  and falls back.
- job/xray_traffic_job.go: union API onlines into the delta-derived active
  set; bump last_online for idle-but-connected clients.
- job/check_client_ip_job.go: API-first IP source with shared enforcement;
  live observations bypass the 30-min stale cutoff; access-log path
  unchanged for older cores.
- service/setting.go: GetIpLimitEnable always true; new accessLogEnable
  default for features that genuinely read the access log.

Frontend:
- Client form split into Basic and Config tabs; IP Limit and IP Log no
  longer gated on access log; compact Auto Renew next to Start After First
  Use; tabBasic/tabConfig added to all 13 locales.
- Xray logs button on the dashboard now gated on accessLogEnable.

internal/xray/api_e2e_test.go

@@ -53,8 +53,12 @@ func TestXrayAPI_E2E(t *testing.T) {
 				map[string]any{"type": "field", "inboundTag": []string{"api"}, "outboundTag": "api"},
 			},
 		},
-		"policy": map[string]any{},
-		"stats":  map[string]any{},
+		"policy": map[string]any{
+			"levels": map[string]any{
+				"0": map[string]any{"statsUserOnline": true},
+			},
+		},
+		"stats": map[string]any{},
 	}
 	cfgBytes, err := json.MarshalIndent(cfg, "", "  ")
 	if err != nil {
@@ -130,6 +134,19 @@ func TestXrayAPI_E2E(t *testing.T) {
 		t.Fatalf("missing inbound error not matched by IsMissingHandlerErr: %q", err)
 	}
 
+	// --- online-stats API ---
+	// statsUserOnline is enabled in the policy above; with no client
+	// connections the call must succeed and return an empty set. This proves
+	// the GetUsersStats plumbing against a real core (an older binary would
+	// return Unimplemented here — see IsUnimplementedErr).
+	online, err := api.GetOnlineUsers()
+	if err != nil {
+		t.Fatalf("GetOnlineUsers: %v", err)
+	}
+	if len(online) != 0 {
+		t.Fatalf("expected no online users on an idle core, got %+v", online)
+	}
+
 	// --- routing (rules + balancers replace) ---
 	newRouting := []byte(`{
 		"domainStrategy": "AsIs",