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/lib/i18n/languages.ts

@@ -1,7 +1,7 @@
 export const LANGUAGE_STORAGE_KEY = "proxmenux-ui-language"
 export const DEFAULT_LANGUAGE = "en"
 
-export type LanguageCode = "en" | "es" | "fr" | "de" | "it" | "pt" | "sk"
+export type LanguageCode = "en" | "es" | "fr" | "de" | "it" | "pt" | "sk" | "sv"
 
 export type LanguageStatus = "complete" | "partial" | "needs-translation"
 
@@ -20,6 +20,7 @@ export const SUPPORTED_LANGUAGES: SupportedLanguage[] = [
   { code: "de", englishName: "German", nativeName: "Deutsch", status: "needs-translation" },
   { code: "it", englishName: "Italian", nativeName: "Italiano", status: "needs-translation" },
   { code: "pt", englishName: "Portuguese", nativeName: "Português", status: "needs-translation" },
+  { code: "sv", englishName: "Swedish", nativeName: "Svenska", status: "needs-translation" },
 ]
 
 export function isSupportedLanguage(value: string | null | undefined): value is LanguageCode {