fix: use local date for daily seed instead of UTC

toISOString() returned UTC date, causing the daily game to reset at
UTC midnight rather than local midnight. Users in UTC+ timezones
would see yesterday's completed game until the UTC date rolled over.

web/lib/champion-data.js

@@ -70,9 +70,13 @@ function hashString(str) {
   return hash >>> 0;
 }
 
-/** Get today's date string for daily seed */
+/** Get today's local date string for daily seed */
 export function getTodaySeed() {
-  return new Date().toISOString().slice(0, 10);
+  const now = new Date();
+  const year = now.getFullYear();
+  const month = String(now.getMonth() + 1).padStart(2, "0");
+  const day = String(now.getDate()).padStart(2, "0");
+  return `${year}-${month}-${day}`;
 }
 
 /** Get champion image URL from Data Dragon CDN */