feat(amneziawg): make the Xray TPROXY bridge a per-inbound opt-in

Addresses Finding 10 from the automated PR review: an always-on TPROXY
bridge makes every AmneziaWG tunnel hard-depend on Xray being up (all
traffic, including DNS, drops whenever Xray restarts), and forces a full
awg-quick down+up bounce on any client add/remove/re-IP, permanently
losing the syncconf fast path.

Adds ServerSettings.RouteThroughXray (off by default):

- defaultPostUpDown only emits the TPROXY/policy-route rules when it's
  on; a plain AmneziaWG tunnel now has zero Xray dependency out of the
  box.
- structuralFingerprint covers it (toggling it changes whether PostUp/
  PostDown contain any TPROXY rules at all -- structural, not a
  per-peer host-rule). hostRulesFingerprint's IPv4 tracking is now
  itself conditional on RouteThroughXray (and IPv6 tracking on
  IPv6Enabled), so an instance that never uses either keeps the
  syncconf fast path for a plain peer re-IP.
- injectAmneziawgEgress only creates a bridge for inbounds that opted
  in; checkAmneziawgEgressConflict (the Finding-7 fix) now parses each
  candidate through InstanceFromInbound so a non-routed inbound's port
  is correctly never treated as reserved.
- New inbound-level Switch in the AmneziaWG form; the actual outbound
  decision is still made entirely through the panel's stock Routing
  page, same as before -- only whether the bridge exists at all is now
  a choice.

Translation keys added to all 13 locales in the same commit this time,
not backfilled later (see Finding 9's lesson).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/public/openapi.json

@@ -2862,6 +2862,10 @@
           "publicKey": {
             "type": "string"
           },
+          "routeThroughXray": {
+            "description": "RouteThroughXray turns on this inbound's TPROXY-into-Xray bridge; see\nInstance.RouteThroughXray for what that means. Off by default.",
+            "type": "boolean"
+          },
           "s1": {
             "type": "integer"
           },