Mobile: real swipe physics, pinch zoom, touch-first lightbox

- swipe sideways to browse (track follows the finger, FLIP settle),
  flick up/down to vote Tinder-style with ghost fly-off and the next
  photo zooming in behind; votes and navigation commit synchronously on
  release — animations are pure decoration, so closing or keypresses
  mid-animation can't drop or duplicate an action
- two fingers pinch-zoom the photo (springs back, never votes)
- voting the last photo fades the modal out over the gallery
- gesture engine extracted to useSwipe: per-move motion via direct DOM
  writes (no React render per touchmove), named threshold constants
- touch devices: selection checkmarks always visible, nav arrows yield
  to swiping, finger-sized vote/rating targets, safe-area padding

frontend/src/pages/SharePage.jsx

@@ -43,6 +43,12 @@ export default function SharePage() {
   const visibleSelected = visible.filter((p) => selected.has(p.id))
   const sumBytes = (list) => list.reduce((sum, p) => sum + p.size_bytes, 0)
   const lightbox = useLightbox(visible)
+  // Voting the last photo ends the run: fade the modal out over the gallery
+  // instead of cutting hard.
+  const [lightboxFading, setLightboxFading] = useState(false)
+  useEffect(() => {
+    if (lightbox.index < 0) setLightboxFading(false)
+  }, [lightbox.index])
 
   const load = useCallback(
     () => api(`/api/share/${token}`).then(setView).catch((e) => setError(e.message)),
@@ -114,6 +120,8 @@ export default function SharePage() {
           : `↺ ${photo.filename} cleared`,
     )
     if (next) lightbox.show(next.id)
+    // Last photo voted: the run is done — fade back to the gallery.
+    else setLightboxFading(true)
   }
 
   const keyActions = [
@@ -171,8 +179,8 @@ export default function SharePage() {
         <h1>{view.album_name}</h1>
         {view.album_description && <p className="muted">{view.album_description}</p>}
         <p className="muted">
-          {photos.length} photo{photos.length === 1 ? '' : 's'} · click a photo to view, rate and
-          tag · press <kbd>?</kbd> in the viewer for shortcuts
+          {photos.length} photo{photos.length === 1 ? '' : 's'} · tap a photo to view, rate and
+          tag · swipe ↑ to accept, ↓ to reject · <kbd>?</kbd> shows keyboard shortcuts
         </p>
         {photos.length > 0 && (
           <div className="filter-bar">
@@ -242,6 +250,15 @@ export default function SharePage() {
           onClose={lightbox.close}
           onNav={lightbox.openAt}
           actions={keyActions}
+          closing={lightboxFading}
+          onClosed={() => {
+            setLightboxFading(false)
+            lightbox.close()
+          }}
+          gestures={{
+            up: (p) => voteAndAdvance(p, 'accept'),
+            down: (p) => voteAndAdvance(p, 'reject'),
+          }}
           footer={(p) => (
             <div className="client-footer">
               <Thumbs value={p.my_verdict} onChange={(v) => setVerdict(p, v)} />