TestLangCompleteness uses reflection to walk the i18n struct and fail
on any empty string field, catching incomplete translations at CI time.
Running it immediately found three missing strings in lang_jp.go
(playtime, timer.enabled, timer.disabled), which are now filled in.
CHANGELOG updated with i18n refactor, FR/ES languages, and the new test.
The config package used `package _config` with a leading underscore,
which is unconventional in Go. Rename to `package config` (matching the
directory name) and use `cfg` as the standard import alias across all
93 importing files.
Port test files from v9.2.x-stable branch to increase channelserver
coverage from 13.8% to 25.6% (556 tests passing).
Adapted all files to main's struct definitions: config import alias,
Airou/CatDefinition rename, packet field mismatches, Raviente struct
differences, and maxPlayers defaults. Removed tests referencing
production code not yet on main (Player, FestivalColour, etc.).
Excluded handlers_register_test.go (Raviente completely redesigned).
