fix: update votedBooks state immediately on swipe so history drawer isn't empty

doAction() was fire-and-forgetting the POST but never updating the client-side
votedBooks array. History was only populated from SSR data.votedBooks (loaded
at page init), so any votes cast during the current session were invisible in
the drawer until a full page reload. Now we prepend/replace an entry in
votedBooks optimistically the moment a swipe action fires.

ui/src/routes/discover/+page.svelte

@@ -239,6 +239,16 @@
 			body: JSON.stringify({ slug: book.slug, action })
 		});
 
+		// Optimistically add/update the history list so the drawer shows it immediately.
+		// If this slug was already voted (e.g. swiped twice via undo+re-swipe), replace it.
+		const existing = votedBooks.findIndex((v) => v.slug === book.slug);
+		const entry: VotedBook = { slug: book.slug, action, votedAt: new Date().toISOString(), book };
+		if (existing !== -1) {
+			votedBooks = [entry, ...votedBooks.filter((_, i) => i !== existing)];
+		} else {
+			votedBooks = [entry, ...votedBooks];
+		}
+
 		// Fly out
 		transitioning = true;
 		const target = flyTargets[action];