feat: add System theme option that follows OS color scheme

Adds a 'System' option to the theme switcher that automatically uses
light or dark theme based on the OS prefers-color-scheme setting.
Dynamically updates when the OS preference changes (e.g. scheduled
dark mode). i18n labels added for EN/FR.

src/lib/i18n.ts

@@ -113,6 +113,7 @@ const en = {
   'theme.title': 'Theme',
   'theme.mode': 'Mode',
   'theme.accent': 'Accent',
+  'theme.system': 'System',
   'theme.dark': 'Dark',
   'theme.light': 'Light',
   'theme.oled': 'OLED',
@@ -218,6 +219,7 @@ const fr: Record<keyof typeof en, string> = {
   'theme.title': 'Thème',
   'theme.mode': 'Mode',
   'theme.accent': 'Accent',
+  'theme.system': 'Système',
   'theme.dark': 'Sombre',
   'theme.light': 'Clair',
   'theme.oled': 'OLED',