feat(inbounds): add Port-with-Fallback inbound type

Adds a new "portfallback" protocol that emits as a VLESS-TLS inbound
under the hood but is paired with a sidecar table of child inbounds.
Panel auto-builds settings.fallbacks at Xray-config-gen time from the
sidecar — each child's listen+port becomes the fallback dest, with
SNI/ALPN/path/xver match criteria pulled from the row. No more typing
loopback ports by hand or keeping settings.fallbacks in sync.

Backend: new FallbackService (Get/SetChildren, BuildFallbacksJSON);
two new routes (GET/POST /panel/api/inbounds/:id/fallbackChildren);
xray.GetXrayConfig injects fallbacks for PortFallback inbounds; the
inbound model emits protocol="vless" so Xray accepts the config.

Frontend: PORTFALLBACK joins the protocol dropdown; selecting it
shows the standard VLESS controls plus a Fallback Children table
(inbound picker + per-row SNI/ALPN/path/xver). Children are loaded
on edit and replaced atomically on save.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/translation/en-US.json

@@ -250,6 +250,12 @@
       "node": "Node",
       "deployTo": "Deploy to",
       "localPanel": "Local panel",
+      "portFallback": {
+        "title": "Fallback children",
+        "help": "Pick inbounds that should catch traffic this VLESS-TLS inbound does not match. Each child must listen on 127.0.0.1 to receive forwarded connections.",
+        "child": "Inbound",
+        "path": "Path"
+      },
       "protocol": "Protocol",
       "port": "Port",
       "portMap": "Port Mapping",