fix: responsive header with hamburger menu for mobile

- Desktop: single row with logo, nav links, QuickAdd, New note button
- Mobile: logo + QuickAdd + hamburger icon → dropdown menu
- Improved QuickAdd sizing for small screens

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

src/components/quick-add.tsx

@@ -188,8 +188,8 @@ export function QuickAdd() {
               onFocus={() => setIsExpanded(true)}
               onPaste={handlePaste}
               className={cn(
-                'w-48 transition-all duration-200',
-                isExpanded && 'w-72'
+                'w-24 xs:w-32 sm:w-48 transition-all duration-200',
+                isExpanded && 'w-40 xs:w-48 sm:w-72'
               )}
               disabled={isLoading}
             />
@@ -202,29 +202,29 @@ export function QuickAdd() {
           type="button"
           onClick={toggleMultiline}
           className={cn(
-            'inline-flex items-center justify-center rounded-lg border bg-background p-2',
+            'inline-flex items-center justify-center rounded-lg border bg-background p-1.5 sm:p-2',
             'hover:bg-accent hover:text-accent-foreground',
             'transition-colors',
             isMultiline && 'bg-accent text-accent-foreground'
           )}
           title={isMultiline ? 'Modo línea' : 'Modo multilínea'}
         >
-          <Text className="h-4 w-4" />
+          <Text className="h-3 w-3 sm:h-4 sm:w-4" />
         </button>
         <button
           type="submit"
           disabled={!value.trim() || isLoading}
           className={cn(
-            'inline-flex items-center justify-center rounded-lg border bg-background p-2',
+            'inline-flex items-center justify-center rounded-lg border bg-background p-1.5 sm:p-2',
             'hover:bg-accent hover:text-accent-foreground',
             'disabled:pointer-events-none disabled:opacity-50',
             'transition-colors'
           )}
         >
           {isLoading ? (
-            <Loader2 className="h-4 w-4 animate-spin" />
+            <Loader2 className="h-3 w-3 sm:h-4 sm:w-4 animate-spin" />
           ) : (
-            <Plus className="h-4 w-4" />
+            <Plus className="h-3 w-3 sm:h-4 sm:w-4" />
           )}
         </button>
       </form>