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>

index.html

@@ -42,6 +42,12 @@
             </div>
         </div>
 
+        <!-- Examples Dropdown -->
+        <div class="toolbar-dropdown" id="examples-section">
+            <button class="dropdown-toggle">Examples <span class="dropdown-arrow">▾</span></button>
+            <div class="dropdown-menu" id="examples-menu"></div>
+        </div>
+
         <button class="action-btn sim-btn" id="sim-btn">Waveform</button>
         <div class="toolbar-right">
             <button class="action-btn export-btn" id="export-btn" title="Export circuit">↓ Export</button>