feat(i18n): add Swedish support + include Slovak in auto-fill defaults

Two related changes to the translation infrastructure:

1. Add Swedish (sv) as a supported locale in both the CLI and Monitor
   translation systems. Inspired by PR #121, which arrived before the
   current auto-generated cache layout existed.

2. Include Slovak (sk) in the default auto-fill target set. Guardrail #1
   in build_i18n_messages.py never overwrites a key whose target value
   differs from EN, so Vaso73's curated Slovak strings stay intact —
   auto-translation only fills keys that are still on the English
   fallback. Trade-off accepted: sk users see decent machine translation
   for new keys immediately instead of raw English while human curation
   catches up, and Vaso73 keeps full ownership of the wording via
   follow-up PRs.

Changes:
- AppImage/lib/i18n/languages.ts: add "sv" to LanguageCode + register
  in SUPPORTED_LANGUAGES.
- AppImage/messages/sv/common.json: stub — will be filled by the i18n
  workflow on the local bootstrap.
- .github/scripts/build_translation_cache.py: add "sv" to
  DEFAULT_LANGUAGES.
- .github/scripts/build_i18n_messages.py: add "sk" and "sv" to
  DEFAULT_LANGUAGES; update comments to explain the sk decision.
- .github/workflows/build-i18n-messages.yml: default input includes
  sk + sv; comments updated to match.

AppImage/messages/sv/common.json

@@ -0,0 +1,3 @@
+{
+  "_meta": "Community translation target. Copy keys from ../en/common.json and translate the values. Placeholders like {vmid} must stay unchanged."
+}