Complete nav rewrite: native details/summary, zero JS state, zero transforms

- Mobile nav: uses <details>/<summary> for toggle, no useState, no useEffect
- Nav dropdown: uses <details>/<summary>, no useState, no event listeners
- Desktop: separate div with desktop nav items (hidden on mobile)
- Mobile: hamburger dropdown with absolute positioned panel
- CSS: removed ::after pseudo-element with transform, replaced hover
  with background-color only, added details[open] CSS rules
- No backdrop-filter, no will-change, no transition-all, no GPU hacks
- Works on every device without JS state management

src/app/globals.css

@@ -208,25 +208,14 @@ summary,
 		transition: color 0.15s ease-in-out, background-color 0.15s ease-in-out;
 		border-radius: 0.5rem;
 	}
-	.nav-item::after {
-		content: "";
-		position: absolute;
-		bottom: 0;
-		left: 50%;
-		width: 60%;
-		height: 3px;
-		border-radius: 3px 3px 0 0;
-		background-color: var(--color-primary);
-		transform: translateX(-50%) scaleX(0);
-		transition: transform 0.2s ease-in-out;
-	}
-	.nav-item:hover::after {
-		transform: translateX(-50%) scaleX(1);
-	}
 	.nav-item:hover {
 		color: var(--color-primary-readable, var(--color-primary));
 		background-color: color-mix(in srgb, var(--color-primary) 8%, transparent);
 	}
+	.nav-item[aria-current="page"] {
+		color: var(--color-primary-readable, var(--color-primary));
+		background-color: color-mix(in srgb, var(--color-primary) 10%, transparent);
+	}
 	.dropdown-item {
 		@apply block py-2.5 px-4 text-sm font-semibold rounded-lg mx-1;
 		color: var(--color-dropdown-text-readable, var(--color-text-readable));
@@ -1076,9 +1065,6 @@ tbody tr:hover {
 		height: auto;
 		min-height: 44px;
 	}
-	.nav-item::after {
-		display: none;
-	}
 	.dropdown-item {
 		font-size: 13px;
 		padding: 0.6rem 0.85rem;
@@ -1135,3 +1121,14 @@ body:has([data-admin]) {
 	--color-input: var(--admin-border);
 	--color-ring: var(--admin-accent);
 }
+
+/* ── Details/summary open states for nav ──────────────────── */
+details[open] > summary .details-open\:hidden {
+	display: none;
+}
+details[open] > summary .details-open\:block {
+	display: block;
+}
+details[open] > summary .details-open\:rotate-180 {
+	transform: rotate(180deg);
+}