Add i18n support and translations (ko, ja, zh_CN, zh_TW)

[NEWBIE0413]

Summary

The shell already wraps user-facing strings in qsTr(), but there was no
mechanism to actually load translations — QuickShell drives the shell with a
bare QQmlEngine and never installs a QTranslator, so qsTr() strings always
rendered in English regardless of locale, and there were no translation files.

This PR adds the missing i18n infrastructure plus initial translations.

Infrastructure (language-agnostic)

• TranslationLoader installs a QTranslator for the current system locale,
  searching the per-user data dir (~/.local/share/caelestia/translations) then
  the system install dir.
• A custom QQmlEngineExtensionPlugin for Caelestia.Internal overrides
  initializeEngine() to load translations at startup (then retranslate()).
• CMake compiles translations/*.ts → .qm via lrelease and installs them
  where the loader looks. qml_module gains an optional PLUGIN_SOURCE arg so a
  module can supply a hand-written plugin entry point.
• A few previously-hardcoded UI strings (weather tab, lyric menu, keyboard-layout
  tooltip) are now wrapped in qsTr().

Graceful no-op when no matching .qm exists — untranslated locales keep the
English source strings. Adding a language is just dropping in
translations/caelestia_<locale>.ts.

Translations (638 strings each)

• Korean (ko) — reviewed by a native speaker.
• Japanese (ja), Simplified Chinese (zh_CN), Traditional Chinese (zh_TW)
  — machine-translated drafts. Community review by native speakers is very
  welcome to refine wording; these are functional but not yet natively proofread.

Maintainers: if you'd prefer to merge only the reviewed Korean + infrastructure
and take the other languages separately (or wait for native review), happy to
split — just let me know.

Test Plan

• Clean full build (extras;plugin;shell) succeeds; all four caelestia_*.qm are produced.
• Placeholder integrity checked across all 4 files (0 mismatches, 0 unfinished, 0 empty).
• Isolated run (qs -p) loads the matching .qm for the system locale (verified ko + ja),
  reaches Configuration Loaded, no crash.
• Non-translated locale (e.g. en_US) logs a graceful fallback to source strings.
• All .ts compile cleanly with lrelease (638 finished, 0 unfinished each).

[hdcy]

Really excited to see this! Native i18n support would be a huge improvement — especially for Chinese users who'd love to get proper translations without relying on external scripts. Looking forward to seeing this land. Thanks for the great work!

[NEWBIE0413]

Thank you! As a Korean user myself, I'd love to see this PR land too 🙂

Quick status update: I've just rebased onto the latest main (it had moved quite a bit) and fixed the CI failures — the branch is now mergeable and checks are re-running.

On the translations: Korean (ko) is reviewed by me as a native speaker, while Japanese and both Chinese variants (zh_CN, zh_TW) are machine-translated drafts — native-speaker review is very welcome to refine the wording. The infrastructure itself is language-agnostic, so adding more languages is just dropping in another .ts file.

[MiguxJS]

Hi! I'm really excited about this i18n infrastructure. I'd love to contribute by creating and maintaining the official Spanish (es) translation for the shell once this lands! Let me know if I can start working on it. 😄