feat(auto-theme): add accent-first cascade, overlay spinner, and failure modal

Source options: System Colors (accent -> wallpaper fallback), Wallpaper
Sync, Custom Image. Selecting System shows fullscreen overlay while
generating. On failure: modal recommends another theme or custom image,
falls back to Dark immediately.

Store cascade logic: tryAccentColorTheme() first (with wallpaper
backgrounds + system accent override), then generateFromWallpaper(),
then throw for caller to handle.

cc

src/lib/i18n/locales/fr.json

@@ -662,7 +662,8 @@
       "autoTheme": "Theme automatique",
       "autoThemeDesc": "Genere un theme de couleurs a partir de votre fond d'ecran ou d'une image personnalisee",
       "autoThemeSource": "Source",
-      "autoThemeSourceWallpaper": "Fond d'ecran systeme",
+      "autoThemeSourceSystem": "Couleurs systeme",
+      "autoThemeSourceWallpaper": "Wallpaper Sync",
       "autoThemeSourceImage": "Image personnalisee",
       "autoThemeSelectImage": "Selectionner une image...",
       "autoThemeGenerating": "Generation du theme...",
@@ -670,6 +671,8 @@
       "autoThemeDetectedDE": "Detecte : {de}",
       "autoThemeNoWallpaper": "Impossible de detecter le fond d'ecran systeme",
       "autoThemeApplied": "Theme automatique applique",
+      "autoThemeRegenerate": "Regenerer",
+      "autoThemeFailedHint": "Nous vous recommandons de choisir un autre theme ou d'en generer un a partir d'une image personnalisee.",
       "accentColor": "Couleur d'accent",
       "language": "Langue",
       "fontFamily": "Police",