fix(cbt): Periodic Table image clarity and reliable full-screen exit

src/components/CBT/PeriodicTable.tsx

@@ -23,14 +23,14 @@ export function PeriodicTable({ isOpen, onClose }: PeriodicTableProps) {
              </button>
           </div>
         </div>
-        <div className="flex-1 bg-white p-4 flex items-start justify-start relative overflow-auto">
-          <div className="relative w-[1800px] h-[1100px] shrink-0">
-             {/* Using a reliable wikimedia SVG for Periodic Table as a reference */}
+        <div className="flex-1 bg-white p-4 flex items-center justify-center overflow-auto">
+          <div className="relative min-w-[1000px] w-full h-full flex items-center justify-center">
              <Image 
-                src="https://upload.wikimedia.org/wikipedia/commons/4/4d/Periodic_table_large.svg"
+                src="https://sciencenotes.org/wp-content/uploads/2021/01/PeriodicTable2021-Color.png"
                 alt="Periodic Table"
-                fill
-                className="object-contain"
+                width={1920}
+                height={1080}
+                className="object-contain w-full h-full"
                 unoptimized
              />
           </div>