feat: add Examples dropdown with pre-built circuits

Add 4 example circuits accessible from a new toolbar dropdown:
- SR Flip-Flop (NOR) — basic set-reset latch
- SR Flip-Flop (NAND) — active-low variant
- D Latch (1-bit Memory) — gated latch with enable
- D Flip-Flop (Master-Slave) — edge-triggered with CLK

Each example shows name + description in the dropdown and loads
the full circuit with proper gate placement and wiring.

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

css/style.css

@@ -217,6 +217,47 @@ body {
     background: #252540;
 }
 
+/* Example buttons */
+.example-btn {
+    width: 100%;
+    text-align: left;
+    padding: 8px 12px;
+    background: transparent;
+    border: 1px solid transparent;
+    border-radius: 5px;
+    color: #ccc;
+    cursor: pointer;
+    transition: all 0.15s;
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
+
+.example-btn:hover {
+    background: #1a1a3a;
+    border-color: #00e599;
+}
+
+.example-name {
+    font-size: 12px;
+    font-weight: 600;
+    color: #00e599;
+}
+
+.example-btn:hover .example-name {
+    color: #00ff99;
+}
+
+.example-desc {
+    font-size: 10px;
+    color: #666;
+    line-height: 1.3;
+}
+
+#examples-menu {
+    min-width: 260px;
+}
+
 /* ==================== Canvas ==================== */
 #canvas {
     position: fixed;