feat: redesign admin panel with Lucide icons and modern styling

- Add lucide-react for SVG icons throughout the admin
- Redesign sidebar with gradient background, icons per nav item, and sticky layout
- Redesign topbar with cleaner user info display
- Redesign dashboard with icon-backed stat cards, gradient progress bars, activity feed
- Update AdminNavLink with icon support and new active state styling
- Improve table styling in admin-page CSS (rounded corners, hover, spacing)
- Clean up unused admin CSS

src/app/globals.css

@@ -436,7 +436,7 @@ tbody tr:hover {
 }
 
 /* ── Admin panel (Housekeeping) ─────────────────────────────────
-   Page content wrapper — table card styling and h1 formatting. */
+   Page content wrapper — card-style tables and consistent spacing. */
 .admin-page main > h1:first-child,
 .admin-page > h1:first-child {
   margin-top: 0;
@@ -447,33 +447,35 @@ tbody tr:hover {
   border-bottom: 2px solid var(--border);
   margin-bottom: 0.85rem;
 }
-/* Carded tables — give the bare admin tables a surface + frame. */
 .admin-page table {
   background: var(--color-surface);
-  box-shadow: var(--shadow-card);
   border: 1px solid var(--border);
-  border-radius: 12px;
+  border-radius: 16px;
   overflow: hidden;
+  box-shadow: var(--shadow-card);
 }
 .admin-page thead th {
-  background: color-mix(in srgb, var(--color-primary) 12%, var(--color-surface));
+  background: color-mix(in srgb, var(--color-primary) 10%, var(--color-surface));
   color: var(--color-text-muted);
   border-bottom: 1px solid var(--border);
   white-space: nowrap;
+  font-size: 0.7rem;
+  letter-spacing: 0.04em;
+  padding: 0.7rem 0.9rem;
 }
-.admin-page th,
 .admin-page td {
-  padding: 0.6rem 0.8rem;
+  padding: 0.7rem 0.9rem;
+  border-top: 1px solid color-mix(in srgb, var(--color-text-muted) 10%, transparent);
+  font-size: 0.88rem;
 }
 .admin-page tbody tr {
   transition: background 0.12s ease;
 }
 .admin-page tbody tr:hover {
-  background: color-mix(in srgb, var(--color-primary) 7%, transparent);
-}
-.admin-page tbody td {
-  border-top: 1px solid color-mix(in srgb, var(--color-text-muted) 12%, transparent);
+  background: color-mix(in srgb, var(--color-primary) 5%, transparent);
 }
+.admin-page thead tr:first-child th:first-child { border-radius: 16px 0 0 0; }
+.admin-page thead tr:first-child th:last-child { border-radius: 0 16px 0 0; }
 /* ── Public design system (faithful to the atom theme) ──────────
    content-card = surface card with a primary-tinted header (icon +
    title + subtitle) and a padded body — the signature atom block. */