feat: add font size toggle functionality

- Implemented a FontSizeToggle component in HomePageClient for adjusting task font sizes in kanban views.
- Updated TaskCard to apply dynamic font size classes based on user preferences.
- Enhanced user preferences to include font size settings, defaulting to 'medium'.
- Modified TODO.md to mark the font size toggle task as complete.

components/ui/FontSizeToggle.tsx

@@ -0,0 +1,42 @@
+'use client';
+
+import { useUserPreferences } from '@/contexts/UserPreferencesContext';
+
+export function FontSizeToggle() {
+  const { preferences, toggleFontSize } = useUserPreferences();
+  
+  // Icône pour la taille de police
+  const getFontSizeIcon = () => {
+    switch (preferences.viewPreferences.fontSize) {
+      case 'small':
+        return 'A';
+      case 'large':
+        return 'A';
+      default:
+        return 'A';
+    }
+  };
+  
+  const getFontSizeScale = () => {
+    switch (preferences.viewPreferences.fontSize) {
+      case 'small':
+        return 'text-xs';
+      case 'large':
+        return 'text-lg';
+      default:
+        return 'text-sm';
+    }
+  };
+
+  return (
+    <button
+      onClick={toggleFontSize}
+      className="flex items-center gap-2 px-3 py-1.5 rounded-md text-sm font-mono transition-all bg-[var(--card)] text-[var(--muted-foreground)] border border-[var(--border)] hover:border-[var(--primary)]/50 hover:text-[var(--primary)]"
+      title={`Font size: ${preferences.viewPreferences.fontSize} (click to cycle)`}
+    >
+      <span className={`font-mono font-bold ${getFontSizeScale()}`}>
+        {getFontSizeIcon()}
+      </span>
+    </button>
+  );
+}