Fix off-by-one in rollback button placement

A rollback button on event E restores the state just after E. So the
button on the latest event was a no-op (you're already there), while the
scene-start event -- a valid earliest target -- correctly has one. The
visible set was effectively shifted one event late.

Hide the button on whichever event sits at the current position (the
rollback head if set, else the latest checkpoint, now supplied by the
backend as state.latest_checkpoint_id since the frontend's own reduce
over the async-populated accumulator wasn't reliably reactive). In live
play the latest event loses its redundant button; when rolled back, the
"you are here" event has none while earlier events show rollback and
later ones show redo.

Verified in a browser across live, rolled-back, and redo states.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/EventLog.svelte

@@ -42,6 +42,10 @@
     // When set, the game is at a rolled-back state; events past this checkpoint are the
     // greyed, still-undoable future. Rolling forward to one of them redoes the rollback.
     $: head = state.game?.rollback_head_checkpoint_id ?? null;
+    // The point in time the game is currently at: the rollback head if set, otherwise
+    // the newest checkpoint (supplied by the backend). A button targeting it would be a
+    // no-op, so the "you are here" event — and, in live play, the latest event — has none.
+    $: currentPos = head !== null ? head : (state.latest_checkpoint_id ?? 0);
 
     $: mergePolled(state.events);
 
@@ -175,7 +179,7 @@
                         <span class="log-icon">{KIND_ICONS[event.kind] || KIND_ICONS.info}</span>
                         <span class="log-message">{event.message}</span>
                         <span class="log-time">{formatTime(event.timestamp)}</span>
-                        {#if canRollback && event.checkpoint_id > 0}
+                        {#if canRollback && event.checkpoint_id > 0 && event.checkpoint_id !== currentPos}
                             <button
                                 class="rollback-btn"
                                 title={isFuture ? 'Redo forward to just after this event' : 'Roll the game back to just after this event'}