feat(i18n): add the Slovak (sk-SK) panel locale (#6889)

Adds a complete Slovak translation of every panel and bot string, registers
sk-SK in the language picker (a browser set to sk now lands on it instead of
English) and loads Ant Design's Slovak locale. The locale count in the READMEs,
docs and contributor guides moves from 13 to 14.

docs/architecture.md

@@ -231,7 +231,7 @@ node heartbeat every 5s, periodic traffic resets (hourly/daily/weekly/monthly).
 │   │   ├── network/          # Custom net listeners (e.g. proxy-protocol aware)
 │   │   ├── session/          # Session/cookie helpers
 │   │   ├── websocket/        # WS hub implementation
-│   │   ├── locale/ + translation/  # i18n middleware + 13 locale JSON catalogs
+│   │   ├── locale/ + translation/  # i18n middleware + 14 locale JSON catalogs
 │   │   ├── entity/           # Shared request/response DTOs
 │   │   └── dist/             # ⚠️ Vite build output, embedded via go:embed (generated — do not hand-edit)
 │   │
@@ -623,7 +623,7 @@ them with a comment starting `@claude review` on a PR or `@claude analyze` on an
   (`FetchCertFingerprint`), `service/node_mtls.go`, and `runtime/tls_client.go`.
 - **Restart is signal-driven.** `main.go` traps SIGHUP to restart panel+sub servers; the
   in-process restart hook (`global.SetRestartHook`) funnels into the same path.
-- **i18n:** backend catalogs in `internal/web/translation/` (13 locales, shared with the
+- **i18n:** backend catalogs in `internal/web/translation/` (14 locales, shared with the
   frontend); frontend wiring in `frontend/src/i18n/`. Persian (`fa_IR`) is a first-class
   locale (Jalali calendar via `persian-calendar-suite`).
 - **Tests live next to code** (`foo.go` ↔ `foo_test.go`), plus golden snapshots in