feat(media): swipe between items in the full-screen viewer (#164)

Touch had no way to move between images except the small arrow buttons.
A one-finger horizontal swipe now goes next/previous, and is inert while
the image is zoomed in (where the same gesture pans). The classifier is
pure and unit-tested: ≥ 60 px horizontal, not mostly vertical, under
800 ms, single finger.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/room/MediaGallery.tsx

@@ -23,6 +23,7 @@ import classNames from 'classnames';
 import { useNearViewport } from '../../hooks/useNearViewport';
 import { useZoom } from '../../hooks/useZoom';
 import { usePan, Pan } from '../../hooks/usePan';
+import { useSwipeNav } from '../../hooks/useSwipeNav';
 import { IEncryptedFile, IImageInfo, IThumbnailContent } from '../../../types/matrix/common';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
@@ -255,7 +256,15 @@ export function Lightbox({
   const { zoom, zoomIn, zoomOut, setZoom } = useZoom(0.2);
   // Pan is only active for a zoomed-in image; usePan resets its offset when this
   // flips false (i.e. back to 1x, on navigation, or on a video).
-  const { pan, cursor, onMouseDown, onTouchStart } = usePan(isImage && zoom !== 1);
+  const zoomedIn = isImage && zoom !== 1;
+  const { pan, cursor, onMouseDown, onTouchStart } = usePan(zoomedIn);
+  // [Gitea #164] Swipe between items on touch — only while not zoomed in,
+  // where the same gesture pans the image instead.
+  const swipe = useSwipeNav(
+    !zoomedIn,
+    useCallback(() => setIndex((i) => Math.min(items.length - 1, i + 1)), [items.length]),
+    useCallback(() => setIndex((i) => Math.max(0, i - 1)), []),
+  );
   const dialogRef = useRef<HTMLDivElement>(null);
   const toggleZoom = useCallback(() => setZoom((z) => (z === 1 ? 2 : 1)), [setZoom]);
 
@@ -446,6 +455,8 @@ export function Lightbox({
             alignItems="Center"
             justifyContent="Center"
             onWheel={handleWheel}
+            onTouchStart={swipe.onTouchStart}
+            onTouchEnd={swipe.onTouchEnd}
             style={{ overflow: 'hidden', padding: config.space.S400 }}
           >
             {index > 0 && (