fix: K key re-jumping to same section instead of advancing

The forward-skip threshold was `cur + 0.5`, which is smaller than
the preroll offset applied when landing on a section. After K placed
the cursor at `s.start - preroll`, the next K press found the same
section again because `s.start > (s.start - preroll) + 0.5` is true
whenever preroll > 0.5. Using `cur + preroll` as the threshold ensures
the current section's start is not > cur + preroll, so only the next
section matches.

web.py

@@ -925,7 +925,7 @@ document.addEventListener('keydown', e => {
     const cur = audio.currentTime;
     let jumped = false;
     for (let i = 0; i < sections.length; i++) {
-      if (sections[i].start > cur + 0.5) {
+      if (sections[i].start > cur + preroll) {
         const s = sections[i];
         audio.currentTime = Math.max(0, s.start - preroll);
         setCutFields(activePlayerIdx, s.start, s.end);