feat: custom scrollbar + expand puzzle levels from 8 to 20

- Custom dark-themed scrollbar (WebKit + Firefox) matching the cyberpunk UI
- Added 12 new levels across new categories:
  - Combinational Logic: MUX, DEMUX, 3-input AND, Majority, Parity
  - Arithmetic: Half Subtractor, 1-bit Comparator
  - Decoders & Encoders: 2-to-4 Decoder, 4-to-2 Encoder, 7-Segment
  - Components: 1-bit ALU
  - Logic Basics: XNOR Gate

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

css/style.css

@@ -1,5 +1,34 @@
 * { margin: 0; padding: 0; box-sizing: border-box; }
 
+/* ==================== Custom Scrollbar ==================== */
+::-webkit-scrollbar {
+    width: 6px;
+    height: 6px;
+}
+
+::-webkit-scrollbar-track {
+    background: transparent;
+}
+
+::-webkit-scrollbar-thumb {
+    background: #2a2a3a;
+    border-radius: 3px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+    background: #00e59966;
+}
+
+::-webkit-scrollbar-corner {
+    background: transparent;
+}
+
+/* Firefox */
+* {
+    scrollbar-width: thin;
+    scrollbar-color: #2a2a3a transparent;
+}
+
 body {
     font-family: 'Segoe UI', system-ui, sans-serif;
     background: #0a0a0f;