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-CN.json

@@ -1755,6 +1755,7 @@
         "tagPlaceholder": "唯一均衡器标签",
         "selector": "选择器",
         "fallback": "Fallback",
+        "cycleTooltip": "循环: {path} → (回到 {start})",
         "expected": "期望",
         "expectedPlaceholder": "最佳节点数",
         "maxRtt": "最大 RTT",
@@ -1764,7 +1765,12 @@
         "balancerDesc": "无法同时使用 balancerTag 和 outboundTag。如果同时使用，则只有 outboundTag 会生效。",
         "costMatch": "标签匹配模式",
         "costValue": "权重",
-        "costRegexp": "正则表达式匹配"
+        "costRegexp": "正则表达式匹配",
+        "balancerDeleteInUse": "无法删除此负载均衡器 — 它被用作以下负载均衡器的备用：{names}",
+        "balancerFallbackCycle": "无法将此负载均衡器设置为备用 — 这会创建循环依赖。",
+        "balancerFallbackInfo": "流量将通过以下路径路由：负载均衡器 → Loopback → 服务器 → 目标负载均衡器 → 出站连接。这会增加一个经过服务器的额外跳转，可能会引入轻微延迟。",
+        "fallbackBalancerHint": "选择另一个负载均衡器作为备用",
+        "reservedPrefix": "_bl_ 前缀保留给内部负载均衡器回环对象"
       },
       "wireguard": {
         "secretKey": "密钥",