feat(languages): add Slovenian (sl) to the supported languages (#995)

Slovenian was missing from SUPPORTED_LANGUAGES, so the Settings picker did not offer it and normalize_language_shortcuts dropped any shortcut bound to it. Add it as a Whisper-only entry (whisper code sl; alphacephei publishes no Slovenian VOSK model), map the Slovenian XKB layout si to it for follow-keyboard-layout, and list it on the website languages page.

Fixes #970

tests/test_system_language.py

@@ -24,6 +24,8 @@ def isolated_config(tmp_path, monkeypatch):
         ("de", "de"),
         ("cz", "cs"),
         ("se", "sv"),
+        ("si", "sl"),
+        ("sk", None),
         ("us", "en-us"),
         ("gb", "en-us"),
         ("xyz", None),
@@ -48,6 +50,8 @@ def test_layouts_map_onto_catalogue_entries(layout, expected):
         ("nb_NO", "no"),
         ("nb_NO.UTF-8", "no"),
         ("nn_NO", "no"),
+        ("sl_SI.UTF-8", "sl"),
+        ("sk_SK.UTF-8", None),
         ("C", None),
         ("POSIX", None),
         ("C.UTF-8", None),