* feat(ios): support per-gateway custom headers for proxied gateway connections
Per-gateway custom request headers (Cloudflare Access-style service tokens)
for gateways behind authenticating reverse proxies. Headers are credentials:
Keychain-stored per gateway stableID, masked in settings UI, injected on the
WebSocket upgrade at connect time via a provider so edits apply on the next
reconnect without re-pairing. Reserved handshake headers and control chars
are sanitized out. TLS pinning and pairing flows unchanged. Push relay and
other non-gateway destinations proven header-free by regression test.
Related: #100698
* chore(ios): sync native i18n inventory for custom header strings
* fix(ios): harden gateway custom headers
* chore(ios): resync native i18n inventory
* fix(ios): validate custom header names
