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.

templates/index.html

@@ -50,6 +50,7 @@
     {{template "contact-button" .}}
     {{template "zoom-toggle-button" .}}
     {{template "shortcuts-button" .}}
+    {{template "bg-photo-toggle" .}}
 
     <!-- ============================================ -->
     <!-- MODALS                                      -->

templates/partials/widgets/bg-photo-toggle.html

@@ -0,0 +1,52 @@
+{{define "bg-photo-toggle"}}
+{{if not .IsProduction}}
+<!-- Background Photo Toggle (Dev Only) -->
+<button
+    id="bg-photo-toggle"
+    class="fixed-btn bg-photo-btn no-print has-tooltip"
+    aria-label="Toggle background photo"
+    data-tooltip="Toggle background photo"
+    onclick="toggleBgPhoto()">
+    <iconify-icon icon="mdi:image-outline" width="24" height="24"></iconify-icon>
+</button>
+<script>
+(function() {
+  var photos = [{{range $i, $p := .BgPhotos}}{{if $i}},{{end}}'{{$p}}'{{end}}];
+  var key = 'bg-photo-enabled';
+  var keyIdx = 'bg-photo-index';
+
+  function pickPhoto() {
+    var idx = Math.floor(Math.random() * photos.length);
+    localStorage.setItem(keyIdx, idx);
+    return photos[idx];
+  }
+
+  function applyPhoto(enabled) {
+    var btn = document.getElementById('bg-photo-toggle');
+    var icon = btn ? btn.querySelector('iconify-icon') : null;
+    if (enabled) {
+      var idx = parseInt(localStorage.getItem(keyIdx), 10);
+      var url = (isNaN(idx) || idx >= photos.length) ? pickPhoto() : photos[idx];
+      document.body.style.setProperty('--bg-photo-url', 'url("' + url + '")');
+      document.body.classList.add('bg-photo');
+      if (icon) icon.setAttribute('icon', 'mdi:image');
+    } else {
+      document.body.classList.remove('bg-photo');
+      if (icon) icon.setAttribute('icon', 'mdi:image-outline');
+    }
+  }
+
+  // Init: pick a random photo each load, restore toggle state
+  pickPhoto();
+  var enabled = localStorage.getItem(key) !== 'false'; // default on
+  applyPhoto(enabled);
+
+  window.toggleBgPhoto = function() {
+    var isOn = document.body.classList.contains('bg-photo');
+    localStorage.setItem(key, !isOn);
+    applyPhoto(!isOn);
+  };
+})();
+</script>
+{{end}}
+{{end}}