fix(gallery): use ref for swipe-start to avoid stale-closure miss (#332)

Found via real-browser verification: with useState the prior commit's
handleTouchEnd captures swipeStart from its render closure, so when
touchstart and touchend fire inside the same React batch (fast swipe,
synthetic events, or a tight render cycle) the end handler reads the
stale null and skips navigation. useRef sidesteps the closure entirely
and is the right primitive for cross-event scratchpad state anyway.

Verified in a 4-photo gallery on mobile-emulation (390x844 touch):
- left swipe (-200px) advances 1/4 → 2/4
- right swipe (+200px) returns 2/4 → 1/4
- 20px swipe (under threshold) does not navigate
- vertical swipe (dy 300, dx 20) does not navigate

frontend/src/components/gallery/PhotoLightbox.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useRef } from 'react';
 import { useDevToolsProtection } from '../../hooks/useDevToolsProtection';
 import { X, ChevronLeft, ChevronRight, Download, ZoomIn, ZoomOut, MessageSquare, Heart, Star, Loader2 } from 'lucide-react';
 import type { Photo } from '../../types';
@@ -47,7 +47,9 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
   const [dragStart, setDragStart] = useState({ x: 0, y: 0 });
   const [dragOffset, setDragOffset] = useState({ x: 0, y: 0 });
   const [touchDistance, setTouchDistance] = useState<number | null>(null);
-  const [swipeStart, setSwipeStart] = useState<{ x: number; y: number; t: number } | null>(null);
+  // Ref (not state) so handleTouchEnd reads the value set by handleTouchStart
+  // even when both fire in the same render batch.
+  const swipeStartRef = useRef<{ x: number; y: number; t: number } | null>(null);
   const [showFeedback, setShowFeedback] = useState(initialShowFeedback);
   const [isSmallScreen, setIsSmallScreen] = useState<boolean>(typeof window !== 'undefined' ? window.innerWidth < 640 : false);
   const [feedbackSettings, setFeedbackSettings] = useState<{
@@ -374,10 +376,10 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
         touch2.clientY - touch1.clientY
       );
       setTouchDistance(distance);
-      setSwipeStart(null);
+      swipeStartRef.current = null;
     } else if (e.touches.length === 1 && zoom <= 1) {
       const t = e.touches[0];
-      setSwipeStart({ x: t.clientX, y: t.clientY, t: Date.now() });
+      swipeStartRef.current = { x: t.clientX, y: t.clientY, t: Date.now() };
     }
   };
 
@@ -399,18 +401,19 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
 
   const handleTouchEnd = (e: React.TouchEvent) => {
     setTouchDistance(null);
-    if (swipeStart && e.changedTouches.length > 0) {
+    const start = swipeStartRef.current;
+    if (start && e.changedTouches.length > 0) {
       const t = e.changedTouches[0];
-      const dx = t.clientX - swipeStart.x;
-      const dy = t.clientY - swipeStart.y;
-      const dt = Date.now() - swipeStart.t;
+      const dx = t.clientX - start.x;
+      const dy = t.clientY - start.y;
+      const dt = Date.now() - start.t;
       // Horizontal swipe: > 50px and dominant over vertical, completed in < 600ms.
       if (Math.abs(dx) > 50 && Math.abs(dx) > Math.abs(dy) * 1.2 && dt < 600) {
         if (dx > 0) goToPrevious();
         else goToNext();
       }
     }
-    setSwipeStart(null);
+    swipeStartRef.current = null;
   };
 
   // Apply protection class to the lightbox container