feat(panel): customers stats strip with 3x-ui depletion semantics (round-2 #1)

GET /api/customers gains a summary object (total/online/depleted/
depletingSoon/disabled/enabled) computed server-side; the customers page
renders it as a six-cell stats strip above the toolbar.

Predicates aligned verbatim with 3x-ui client_paging.go:
- depleted = (limit>0 && used>=limit) || (expiry set && expiry<=now),
  counted regardless of status;
- depletingSoon = active && !depleted && (expiry within expireDiff days
  || remaining traffic < trafficDiff GB), thresholds from new settings
  expireDiff/trafficDiff with 3x-ui's factory default 0 (nobody counts
  until configured);
- enabled = status active; disabled = manual suspensions only.
Online reuses the existing 3-minute presence projection.

Golden diff confined to the additive summary keys; summary_test.go
covers every bucket with thresholds off (soon=0) and on (soon=2).

public/styles.css

@@ -210,6 +210,7 @@ tbody tr.drag-over td{border-top:2px solid var(--primary)}
 
 /* ---------- 服务器页 ---------- */
 .stats{display:grid;grid-template-columns:repeat(3,1fr);background:var(--card);border:1px solid var(--border-soft);border-radius:var(--radius);margin-bottom:14px}
+.stats.six{grid-template-columns:repeat(6,1fr)}
 .stats .stat{padding:14px 22px;border:none;background:transparent}
 .stats .stat .lab{color:var(--text-2);font-size:12.5px;margin-bottom:3px}
 .stats .stat .val{display:flex;align-items:center;gap:8px;font-size:20px;font-weight:600;font-variant-numeric:tabular-nums}
@@ -401,5 +402,5 @@ label>input:focus,label>select:focus,label>textarea:focus{border-color:var(--pri
 .toast{background:var(--pop-bg);border:1px solid var(--border);border-left:3px solid var(--green);color:var(--text);border-radius:8px;box-shadow:var(--pop-shadow);padding:9px 14px;font-size:13px;max-width:360px}
 .toast.error{border-left-color:var(--red)}
 
-@media (max-width:1100px){.stat-grid{grid-template-columns:repeat(2,1fr)}.mid-grid{grid-template-columns:1fr}.info-bar{flex-direction:column;gap:14px}.info-bar .seg{padding:0}.info-bar .seg+.seg{border-left:none}.stats{grid-template-columns:repeat(2,1fr)}}
+@media (max-width:1100px){.stat-grid{grid-template-columns:repeat(2,1fr)}.mid-grid{grid-template-columns:1fr}.info-bar{flex-direction:column;gap:14px}.info-bar .seg{padding:0}.info-bar .seg+.seg{border-left:none}.stats{grid-template-columns:repeat(2,1fr)}.stats.six{grid-template-columns:repeat(3,1fr)}}
 @media (max-width:760px){.sider{display:none}.form-grid{grid-template-columns:1fr}.top-search{display:none}.content{padding:8px}}