feat(balancer): add balancer-to-balancer fallback support (#5586)

* feat(balancer): add balancer-to-balancer fallback support

Xray does not natively support using a balancer as fallbackTag for
another balancer. This feature automates the loopback workaround:
when a user selects a balancer as fallback, the panel generates a
loopback outbound + routing rule in the template.

How it works:
- User picks fallback balancer from dropdown
- Panel creates loopback outbound _bl_{target} + routing rule
- Balancer fallbackTag set to _bl_{target}
- Traffic: Balancer A → loopback _bl_B → routing rule → Balancer B

Key features:
- Dedup: multiple balancers sharing same fallback reuse one loopback
- DFS cycle detection at edit time and on save
- Self-reference guard (cannot select own balancer)
- Delete protection (blocks if used as fallback by others)
- Cleans up routing rules referencing deleted balancers
- Override resolves balancer tags through loopback mechanism
- All live status tags resolved for display
- Internal _bl_ objects filtered from Outbounds/Routing UI
- Backward-compatible with old _bl_ naming format
- Translations for all 13 locales

* fix(review): override regression, save payload sync, i18n completeness

- OverrideBalancer: only resolve to loopback when resolution succeeds,
  pass original target through for plain outbound tags
- onSaveAll: serialize cleaned template before save to ensure the
  healed/cleaned config is what gets persisted
- Add reservedPrefix translation key to all 12 non-English locales
- Restore trailing newlines in all 13 translation JSON files

* fix(test): update balancer form modal tests after cycle-detection guard

The okButtonProps disabled guard (added in 56d5825c) prevents the modal
from firing onOk when the form is invalid. The old tests clicked the
button expecting validation errors to appear, but antd Modal never calls
onOk on a disabled button — causing false failures.

Rewrite to test the actual guard behavior:
- Button starts disabled (empty form)
- Stays disabled with tag only (selector still empty)
- Stays disabled for duplicate tag
- Disabled button does not trigger onConfirm

---------

Co-authored-by: MHSanaei <ho3ein.sanaei@gmail.com>

internal/web/translation/zh-TW.json

@@ -1755,6 +1755,7 @@
         "tagPlaceholder": "唯一均衡器標籤",
         "selector": "選擇器",
         "fallback": "Fallback",
+        "cycleTooltip": "循環: {path} → (回到 {start})",
         "expected": "期望",
         "expectedPlaceholder": "最佳節點數",
         "maxRtt": "最大 RTT",
@@ -1764,7 +1765,12 @@
         "balancerDesc": "無法同時使用 balancerTag 和 outboundTag。如果同時使用，則只有 outboundTag 會生效。",
         "costMatch": "標籤比對模式",
         "costValue": "權重",
-        "costRegexp": "正規表示式比對"
+        "costRegexp": "正規表示式比對",
+        "balancerDeleteInUse": "無法刪除此負載平衡器 — 它被用作以下負載平衡器的備用：{names}",
+        "balancerFallbackCycle": "無法將此負載平衡器設定為備用 — 這會建立循環依賴。",
+        "balancerFallbackInfo": "流量將透過以下路徑路由：負載平衡器 → Loopback → 伺服器 → 目標負載平衡器 → 出站連線。這會增加一個經過伺服器的額外跳轉，可能會引入輕微延遲。",
+        "fallbackBalancerHint": "選擇另一個負載平衡器作為備用",
+        "reservedPrefix": "_bl_ 前綴保留給內部負載均衡器迴圈物件"
       },
       "wireguard": {
         "secretKey": "金鑰",