feat(i18n): downloadable language-pack infrastructure

[mteguhfh]

Overview

Implement a downloadable language-pack system that allows optional locales (such as id-ID) to be installed independently from the StaffDeck application, while Chinese and English remain bundled as the base languages.

Proposed Directory Layout

language-packs/
  manifest.json                # Root manifest listing all available packs
  <locale>/
    manifest.json              # Per-pack metadata
    catalog.json               # Translation key-value pairs

Pack Manifest Fields (language-packs/<locale>/manifest.json)

Field	Description
locale	BCP 47 locale code (e.g., id-ID)
name	English name (e.g., Indonesian)
nativeName	Native name (e.g., Bahasa Indonesia)
version	Pack version (semver)
minAppVersion	Minimum StaffDeck version this pack is compatible with
sourceCatalogHash	SHA-256 hash of the source en.json used to generate this pack
sha256	SHA-256 checksum of the catalog.json file

Runtime Behavior

1. Application starts with zh-CN and en-US bundled as base
2. 2. User selects an uninstalled locale (e.g., id-ID) from the LanguageSwitcher
3. 3. Frontend requests the pack from the backend endpoint (e.g., GET /api/v1/language-packs/<locale>)
4. 4. Backend serves the pack only if minAppVersion matches the running version
5. 5. Frontend verifies the SHA-256 checksum of the downloaded catalog
6. 6. Pack is cached in the application data directory for offline use
7. 7. Fallback order: selected locale → English → Chinese source

First Language Pack: id-ID

The id-ID pack will be the first standalone language pack built from the existing translations at PR #21. Once the loader infrastructure is merged, the translations will be rebased and restructured as the first installable pack.

Out of Scope (separate follow-ups)

• Language pack update/auto-update mechanism
• • Language pack CDN distribution
• • • Community-contributed pack submission process

Tasks

• Agree on manifest and storage contract
• - [ ] Implement backend endpoint: list available packs + download pack
• - [ ] Implement frontend loader: download, verify, cache, load
• - [ ] Implement fallback mechanism: selected locale → English → Chinese
• - [ ] Restructure id-ID as the first installable pack
• - [ ] Update check-i18n.cjs to validate pack format

[mteguhfh]

@fadeoreo Tracking issue created as discussed in PR #21. Feel free to suggest any changes to the manifest contract or runtime behavior before implementation begins.