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/es-ES.json

@@ -1755,6 +1755,7 @@
         "tagPlaceholder": "etiqueta única de balanceador",
         "selector": "Selector",
         "fallback": "Fallback",
+        "cycleTooltip": "Ciclo: {path} → (volver a {start})",
         "expected": "Esperado",
         "expectedPlaceholder": "número óptimo de nodos",
         "maxRtt": "Máx. RTT",
@@ -1764,7 +1765,12 @@
         "balancerDesc": "No es posible utilizar balancerTag y outboundTag al mismo tiempo. Si se utilizan al mismo tiempo, sólo funcionará outboundTag.",
         "costMatch": "Patrón de etiqueta",
         "costValue": "Peso",
-        "costRegexp": "Coincidencia por expresión regular"
+        "costRegexp": "Coincidencia por expresión regular",
+        "balancerDeleteInUse": "No se puede eliminar este balanceador — se usa como respaldo para: {names}",
+        "balancerFallbackCycle": "No se puede establecer este balanceador como respaldo — crearía una dependencia circular.",
+        "balancerFallbackInfo": "El tráfico se enrutará a través de: Balanceador → Loopback → Servidor → Balanceador destino → Conexión saliente. Esto agrega un salto adicional a través del servidor, lo que puede introducir ligeros retrasos.",
+        "fallbackBalancerHint": "Seleccione otro balanceador como respaldo",
+        "reservedPrefix": "El prefijo _bl_ está reservado para objetos loopback internos del balanceador"
       },
       "wireguard": {
         "secretKey": "Llave secreta",