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/01-foundation/_themes.css

@@ -62,6 +62,9 @@
   /* Sidebar (for non-clean theme) */
   --sidebar-bg: #d1d4d2;
 
+  /* Background photo tint — controls how much of the photo shows through */
+  --page-bg-tint: rgba(214, 214, 214, 0.85);
+
   /* Legacy CV content variables - theme-aware overrides */
   --text-dark: #1a1a1a;  /* Dark text for light background */
   --text-gray: #333333;  /* Secondary text for light background */
@@ -116,6 +119,9 @@
   /* Sidebar (for non-clean theme) - darker than light theme but lighter than main content */
   --sidebar-bg: #3a3d3e;
 
+  /* Background photo tint — darker overlay for dark theme */
+  --page-bg-tint: rgba(58, 58, 58, 0.85);
+
   /* Legacy CV content variables - theme-aware overrides */
   --text-dark: #e0e0e0;  /* Light text for dark background */
   --text-gray: #d0d0d0;  /* Secondary text for dark background */
@@ -171,6 +177,9 @@
     /* Sidebar (for non-clean theme) - matches explicit dark theme */
     --sidebar-bg: #3a3d3e;
 
+    /* Background photo tint — darker overlay for dark theme */
+    --page-bg-tint: rgba(58, 58, 58, 0.85);
+
     /* Legacy CV content variables - theme-aware overrides */
     --text-dark: #e0e0e0;  /* Light text for dark background */
     --text-gray: #d0d0d0;  /* Secondary text for dark background */