Add a complete Czech (cs) locale to hermes-webui: full key parity with the
English reference (1642/1642 keys), real Czech translations for all string and
function-valued keys, Slavic plural helpers for tool/worklog summaries, the cs
login-screen locale in api/routes.py, and a dedicated tests/test_czech_locale.py
parity+placeholder+diacritics guard mirroring the other per-locale tests. All
15 locale-count assertions bumped 14->15.
Co-authored-by: ostravajih <ostravajih@users.noreply.github.com>
Full-suite caught 7 more per-locale count tests: open_in_vscode (->13, a
pre-existing one-locale gap unrelated to vi), usage_cache_hit_detail/usage_cached_percent
(->14), and the i18n_tooltips LOCALE_COUNT (13->14). vi carries these keys.
