feat: background photo system — random Lanzarote landscapes behind CV grid

Dev-only toggle button enables/disables photo backgrounds. Photos are
auto-discovered from static/images/backgrounds/ and randomly selected
on each page load. Production is unaffected — no button, no photo.

static/css/04-interactive/_buttons.css

@@ -124,6 +124,39 @@
     color: #27ae60; /* Green icon when at bottom */
 }
 
+/* Background Photo Toggle (Dev Only - above download button) */
+.bg-photo-btn {
+    position: fixed;
+    bottom: 30rem;
+    left: 2rem;
+    width: 50px;
+    height: 50px;
+    background: var(--black-bar, #2b2b2b);
+    color: white;
+    border: none;
+    border-radius: 50%;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
+    transition: all 0.3s ease;
+    z-index: 999;
+    opacity: 0.6;
+}
+
+.bg-photo-btn:hover {
+    opacity: 1;
+    transform: translateY(-3px);
+    box-shadow: 0 6px 16px rgba(0, 0, 0, 0.4);
+    background: #8e6b3e !important; /* Earthy brown — Lanzarote vibes */
+}
+
+.bg-photo-btn.at-bottom {
+    opacity: 1;
+    background: #8e6b3e !important;
+}
+
 /* Download Button (TOP POSITION - now first button after cmd-k removed) */
 .download-btn {
     position: fixed;