feat(themes): add light, light-slate, and light-rose themes

Three light variants mirroring the existing dark set. All use CSS custom
properties so no component changes are needed. Theme dots in the header
and mobile drawer show a separator between dark/light groups; light-theme
swatches get a subtle ring so they're visible on light backgrounds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/messages/fr.json

@@ -160,6 +160,9 @@
   "profile_theme_amber": "Ambre",
   "profile_theme_slate": "Ardoise",
   "profile_theme_rose": "Rose",
+  "profile_theme_light": "Light",
+  "profile_theme_light_slate": "Light Blue",
+  "profile_theme_light_rose": "Light Rose",
   "profile_reading_heading": "Paramètres de lecture",
   "profile_voice_label": "Voix par défaut",
   "profile_speed_label": "Vitesse de lecture",