fix: replace outer <button> with <div> in TimelineTrackContent to resolve nested button hydration error

TimelineTrackContent wrapped its entire content in a <button>, while each
timeline element (ElementInner) also rendered its own <button> for click
handling. This created an invalid HTML structure where <button> was nested
inside <button>, triggering a React hydration error:

  "In HTML, <button> cannot be a descendant of <button>."

The outer <button> in TimelineTrackContent only handled click-to-deselect
and mousedown events — it did not need button semantics. Replacing it with
a <div> preserves the exact same event handling behavior while eliminating
the invalid nesting.

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

apps/web/src/components/editor/panels/timeline/timeline-track.tsx

@@ -63,7 +63,7 @@ export function TimelineTrackContent({
 	});
 
 	return (
-		<button
+		<div
 			className="size-full"
 			onClick={(event) => {
 				if (shouldIgnoreClick?.()) return;
@@ -74,7 +74,6 @@ export function TimelineTrackContent({
 				event.preventDefault();
 				onTrackMouseDown?.(event);
 			}}
-			type="button"
 		>
 			<div className="relative h-full min-w-full">
 				{track.elements.length === 0 ? (
@@ -107,6 +106,6 @@ export function TimelineTrackContent({
 					})
 				)}
 			</div>
-		</button>
+		</div>
 	);
 }