fix: improve backdrop blur visibility on dropdowns and overlays

- Reduce dropdown background opacity from 97% to 80% so backdrop-filter is visible
- Add glass-dropdown CSS class with blur(16px) and saturate(180%)
- Add backdrop blur to mobile nav overlay for smoother feel
- Increase photo lightbox backdrop blur from 2px to 8px
- Add background-blend-mode: overlay to site-bg for richer texture

src/components/navbar-color-picker.tsx

@@ -73,7 +73,7 @@ export function NavbarColorPicker() {
       {mounted && open && (
         <div
           className="absolute right-0 mt-1 min-w-[200px] rounded-xl shadow-xl z-50 py-3 px-2 border backdrop-blur-md"
-          style={{ backgroundColor: "color-mix(in srgb, var(--color-dropdown) 97%, transparent)", borderColor: "color-mix(in srgb, var(--color-text-muted) 10%, transparent)" }}
+          style={{ backgroundColor: "color-mix(in srgb, var(--color-dropdown) 80%, transparent)", borderColor: "color-mix(in srgb, var(--color-text-muted) 10%, transparent)" }}
         >
           <div className="text-xs font-semibold uppercase tracking-wider mb-2 px-2" style={{ color: "var(--color-text-muted)" }}>Navbar kleur</div>
           <div className="grid grid-cols-5 gap-1.5">