feat(i18n): add Traditional Chinese (zh-TW) locale

Adds full Traditional Chinese translation (628 keys, 100% parity with
en.json) and the registration plumbing to make it pickable in the UI.

Registration spans three layers that all needed updating for the locale
to actually be selectable end-to-end:
- static/locales/zh-TW.json (new) — TW vocabulary (儲存/雲端/檔案/偵測),
  uses 「」 corner brackets for in-string quoting
- core/i18n.js: add 'zh-TW' to supportedLocales — without this,
  setLocale('zh-TW') was silently rejected by the whitelist and the
  previous locale stayed active (visible as the "picked 繁中 but the
  setup step still shows 簡中" bug)
- core/languageSelector.js: add 'zh-TW' to AVAILABLE_LOCALES + fallback
- features/auth/auth.js: add 'zh-TW' to ALL_LANGUAGES (🇹🇼 繁體中文)
  and LANGUAGE_TEXTS bootstrap table (used before i18n loads)

Browser detection rewrite (i18n.js + auth.js detectBrowserLanguage):
The previous navigator.language?.substring(0, 2) truncated zh-TW → zh
and routed Traditional Chinese browsers to Simplified. Replaced with
three-tier matching: exact full-tag > Chinese script/region heuristic
(zh-Hant*, zh-{TW,HK,MO}) > primary subtag fallback.

Disambiguates the existing zh entry: "Chinese / 中文" became
"Simplified Chinese / 简体中文".

Drive-by cleanups discovered while wiring up the above:
- Remove dead t() in i18n.js (export uses safeT, no callers of bare t)
- Remove dead fetchUserData() and logout() in auth.js (userMenu.js has
  its own local logout())
- Extract errMessage(unknown→string) and inputVal(id) helpers for the
  catch sites and getElementById('x').value sites that needed TS
  narrowing under checkJs
- Type-annotate module-scope let forms/errors/panels with
  HTMLFormElement and HTMLElement so .addEventListener and .reset()
  resolve under strict
- Drop navigator.userLanguage IE legacy fallback (DOM lib has no field)
- jsconfig.json: drop exactOptionalPropertyTypes (only valid with
  strictNullChecks, which the project deliberately disables)
- .gitignore: ignore docker-compose.override.yml for local bind-mount
  dev workflow

Verified clean before commit: biome ci, tsc --noEmit, i18n key parity
(628/628), HTTP smoke test against running container.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -60,6 +60,9 @@ npm-debug.log
 .env.test.local
 .env.production.local
 
+# Docker Compose local overrides (e.g. dev-only bind mounts)
+docker-compose.override.yml
+
 # Log files
 *.log
 logs/