fix: stabilize drag-to-reorder with layout animation fixes

- Remove transition-all from list items (fights framer-motion layout)
- Add layout="position" to Reorder.Item for proper sibling tracking
- Replace CSS gap with marginBottom (gap confuses layout engine)
- Add explicit short transition duration for snappy reorder

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

src/client/routes/threads/$threadId.tsx

@@ -226,7 +226,7 @@ function ThreadDetailPage() {
 						axis="y"
 						values={displayItems}
 						onReorder={setTempItems}
-						className="flex flex-col gap-2"
+						className="flex flex-col"
 					>
 						{displayItems.map((candidate, index) => (
 							<CandidateListItem