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.

.github/workflows/build-i18n-messages.yml

@@ -10,12 +10,11 @@ name: Build i18n messages
 #
 # Guardrails baked into build_i18n_messages.py:
 #   - Never overwrites a key whose target value differs from EN (i.e.
-#     already translated by a human).
+#     already translated by a human). This is what makes it safe to
+#     include sk in the default set: Vaso73's curated strings are
+#     protected end-to-end; auto only fills keys still on the EN
+#     fallback.
 #   - `{placeholder}` tokens are protected end-to-end.
-#   - `sk` is excluded from the default list — Vaso73's locale is
-#     human-curated and full. Add it back via workflow_dispatch inputs
-#     if you ever want auto-fill for the ~200 keys added post his last
-#     PR (only missing keys will be touched; existing ones are safe).
 #
 # Triggers:
 #   - push to develop touching AppImage/messages/en/common.json
@@ -35,8 +34,8 @@ on:
         type: boolean
         default: false
       languages:
-        description: 'Comma-separated locales. Default: es,de,fr,it,pt (sk skipped).'
-        default: 'es,de,fr,it,pt'
+        description: 'Comma-separated locales. Default: es,de,fr,it,pt,sk,sv (guardrail protects Vaso73 sk).'
+        default: 'es,de,fr,it,pt,sk,sv'
 
 # Prevent two runs from racing on the same branch and fighting over the
 # auto-commit. cancel-in-progress:false because a full first-run may take