Improve Traditional Chinese translations

[dlackty]

What changed

• Improve and modernize Traditional Chinese (zh-TW) translations.
• Use terminology more commonly used in Taiwan (TW):
  • Campaign → 行銷活動
  • Template → 版型
  • Header → 標頭
  • Blocklist → 封鎖名單
  • Account → 帳號
  • Connection → 連線
• Fix inaccurate, outdated, and incomplete translations.
• Restore the missing {error} placeholder in campaigns.invalidCustomHeaders.
• Keep Chinese singular and plural labels identical without unnecessary | separators.

Why

The existing zh-TW translation had complete key coverage, but some entries:

• used inconsistent terminology;
• contained wording uncommon in Taiwan;
• had drifted from the current English source;
• omitted important details or interpolation placeholders.

The missing {error} placeholder also prevented the underlying custom-header validation error from being displayed.

Validation

• Confirmed all 713 English translation keys are present.
• Confirmed there are no extra or empty translations.
• Confirmed interpolation placeholders match the English source.
• Validated the JSON syntax.
• Ran go test ./internal/i18n.
• Ran git diff --check.