feat(webroot): implement multi-language (i18n) support for WebUI

[HSSkyBoy]

Summary

Adds full multi-language (i18n) support to the WebUI, with English,
Traditional Chinese (zh-TW), and Simplified Chinese (zh-CN) translations.

• New i18n.js module: a dictionary-based translation system with
  t(key, fallback) for programmatic lookups and translateDOM() to
  translate all [data-i18n] elements in the DOM.
• Language preference is persisted via localStorage and applied on
  load; switching language reloads the page to fully re-render text.
• Falls back to English for any key missing in the active locale, so a
  partial translation never breaks the UI.
• Technical terms and identifiers (e.g. StrongBox, TEE, schema field
  names like Brand/Device/Serial) are intentionally left
  untranslated to avoid ambiguity for developer-facing/debugging text.
• Covers all views: Profiles, Keyboxes, Keys, System, Logs, and their
  associated dialogs, hints, and error states.

Design notes

• Translations favor natural, idiomatic phrasing over literal
  word-for-word translation, so longer descriptive strings (e.g.
  attestation/harvest failure messages, patch vs. generation mode
  explanations) read clearly to native speakers rather than as
  machine-translated text.
• All three locale dictionaries are kept in sync — no missing or extra
  keys across en / zh-TW / zh-CN.

Testing

• Verified i18n.js loads without syntax errors and all three locales
  expose an identical key set.
• Manually switched languages in the WebUI and confirmed labels, hints,
  and error messages render correctly with no leftover untranslated
  strings (aside from the intentionally-untranslated technical terms).