feat(themes): v2.1.0 - themes personnalisés avec presets light/dark/high-contrast/sepia

- 4 modes: dark, light, high-contrast, sepia (40+ CSS vars chacun)
- Import/export thèmes personnalisés en JSON
- Grid swatches dans les paramètres avec preview
- toggleThemeMode() cycle through all 4 modes
- CustomEvent 'themechange' dispatché
- 18 tests unitaires (structure, generators, colors, i18n keys)
- 439 tests passent au total

frontend/js/ui.js

@@ -177,14 +177,16 @@ export function toggleTheme() {
   // Update the theme toggle label
   const themeLabel = document.getElementById("theme-label");
   if (themeLabel) {
-    themeLabel.textContent = newMode === "dark" ? t('theme.dark') : t('theme.light');
+    const modeLabelMap = { dark: 'theme.dark', light: 'theme.light', 'high-contrast': 'theme.high_contrast', sepia: 'theme.sepia' };
+    themeLabel.textContent = t(modeLabelMap[newMode] || 'theme.dark');
   }
   // Update the theme toggle icon
   const themeBtn = document.getElementById("theme-toggle");
   if (themeBtn) {
     const icon = themeBtn.querySelector("i");
     if (icon) {
-      icon.setAttribute("data-lucide", newMode === "dark" ? "moon" : "sun");
+      const isLight = newMode === "light" || newMode === "sepia";
+      icon.setAttribute("data-lucide", isLight ? "sun" : "moon");
     }
   }
   safeCreateIcons();