fix(jpip): give thumbnail priority over pending scroll drain

On fast local servers the fetch-decode cycle is short enough that
the user is still scrolling when it completes, setting pendingFetch
every cycle.  Since pendingFetch was checked before thumbnailPending,
the thumbnail fetch was indefinitely deferred.  On CDN-hosted servers
the 50-200ms network latency gives the user time to pause, breaking
the cycle — masking the bug in production.

Swap the priority: check thumbnailPending first.  It fires once, adds
~50ms for the coarse overview, then the pending scroll drains normally.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: osamu620

web/jpip_viewer.html

@@ -1481,21 +1481,20 @@ <h3>JPIP Gigapixel Viewer</h3>
   // arrived during the await above, fire one more fetch with the current
   // (freshest) panX/panY/zoom.  No debounce — the user already waited the
   // full network+decode round-trip, they don't need another 60 ms.
-  if (pendingFetch) {
-    pendingFetch = false;
-    queueMicrotask(fetchView);
-    return;
-  }
-  // Kick the one-time thumbnail fetch on idle, after the first successful
-  // main-view paint.  Uses its own busy-lock to coexist with any later
-  // user-driven fetch.  Skip the adjacent-halo prefetch this round —
-  // fetchThumbnail owns the decoder for the next hop; the next pan will
-  // set up a new prefetch anyway.
+  // Thumbnail takes priority over pending scrolls — it's a one-time fetch
+  // and on fast local servers the pendingFetch drain can indefinitely
+  // preempt it (user is still scrolling during the short decode window,
+  // setting pendingFetch every cycle).
   if (thumbnailPending) {
     thumbnailPending = false;
     queueMicrotask(fetchThumbnail);
     return;
   }
+  if (pendingFetch) {
+    pendingFetch = false;
+    queueMicrotask(fetchView);
+    return;
+  }
   // Otherwise: schedule an idle-delay prefetch of the adjacent halo.
   // scheduleFetch() cancels this timer on the next user interaction, so
   // continuous panning never triggers a prefetch (the halo would be