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.

CLAUDE.md

@@ -63,7 +63,7 @@ file locations when it can answer in one hop.
     CPU/memory watchdogs, …); full table in `docs/architecture.md` §5.4.
   - `middleware/`, `entity/`, `global/`, `session/` (CSRF), `network/`,
     `runtime/` (master/sub-node over mTLS), `websocket/`.
-  - `locale/` + `translation/` — i18n, 13 embedded locale JSON files.
+  - `locale/` + `translation/` — i18n, 14 embedded locale JSON files.
 - `frontend/` — React + TS source (see `frontend/CLAUDE.md`).
 - `tools/openapigen/` — Go generator that emits frontend types + Zod/JSON schemas
   into `frontend/src/generated/` from Go structs. The OpenAPI doc itself
@@ -107,7 +107,7 @@ file locations when it can answer in one hop.
   `cd docs && pnpm gen:api` to refresh the MDX under
   `docs/content/docs/en/reference/api/`. `docs-ci.yml` fires only on `docs/**`.
 - A new English i18n key goes in EVERY locale JSON in `internal/web/translation/`
-  (13 files) AND must be referenced from `frontend/src` or Go in the SAME commit —
+  (14 files) AND must be referenced from `frontend/src` or Go in the SAME commit —
   `frontend/src/test/i18n-dead-keys.test.ts` fails both ways. It is a frontend
   test, so run `npm test`, not just `make test-go`. At runtime the frontend falls
   back to en-US; Go (`internal/web/locale/`) returns "" for an unknown key.