feat: unified Catalog page with Location→Category→Subcategory→Product tree

- New /catalog page with tree view: Location (🌍) → Category (📂) → Subcategory (📁) → Product
- Add/delete locations, categories, subcategories, products from one page
- JS-powered subcategory dropdown filtered by category
- Sticky sidebar with Add Location/Category/Product forms
- Responsive grid layout (tree + forms side by side, stacks on mobile)
- Navigation simplified: Catalog replaces separate Locations/Categories/Products
- Old routes still accessible for backward compatibility
- Subcategories table migration (006_subcategories.js)
- subcategory_id column added to products table
- Seed data includes subcategories (VPN, Accounts, Hardware, etc.)

src/admin/public/style.css

@@ -230,10 +230,98 @@ pre { font-size: 0.8rem; white-space: pre-wrap; word-break: break-all; max-width
 .seed-card h2 { margin-bottom: 0.5rem; }
 .seed-card p { margin-bottom: 1rem; color: var(--muted); }
 
+.catalog-grid {
+  display: grid;
+  grid-template-columns: 1fr 360px;
+  gap: 1.5rem;
+  align-items: start;
+}
+
+@media (max-width: 900px) {
+  .catalog-grid { grid-template-columns: 1fr; }
+}
+
+.catalog-tree h2 { margin-bottom: 0.75rem; }
+
+.catalog-forms { position: sticky; top: 1rem; }
+
+.tree-node { margin-left: 0; }
+.tree-location > .tree-children { margin-left: 0; }
+
+.tree-children {
+  margin-left: 1.5rem;
+  border-left: 2px solid var(--border);
+  padding-left: 0.75rem;
+}
+
+.tree-label {
+  display: flex;
+  align-items: center;
+  gap: 0.4rem;
+  padding: 0.35rem 0;
+  flex-wrap: wrap;
+}
+
+.tree-label strong { font-size: 0.95rem; }
+
+.tree-icon { font-size: 1rem; }
+
+.tree-meta {
+  font-size: 0.78rem;
+  color: var(--muted);
+  margin-left: 0.25rem;
+}
+
+.tree-empty {
+  font-size: 0.85rem;
+  color: var(--muted);
+  margin: 0.3rem 0;
+  padding-left: 1.5rem;
+}
+
+.tree-product {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.3rem 0;
+  margin-left: 1.5rem;
+  border-left: none;
+}
+
+.tree-product .tree-name { font-weight: 500; font-size: 0.9rem; }
+
+.tree-add {
+  margin: 0.3rem 0 0.3rem 1.5rem;
+}
+
+.tree-location {
+  background: var(--card);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  padding: 0.75rem 1rem;
+  margin-bottom: 0.75rem;
+}
+
+.tree-location > .tree-children { margin-left: 0; border-left: none; padding-left: 0; }
+
+.tree-node:not(.tree-location) .tree-children {
+  margin-left: 1rem;
+  border-left: 2px solid var(--border);
+  padding-left: 0.5rem;
+}
+
+.form-row {
+  display: flex;
+  gap: 0.5rem;
+}
+
+.form-row input { flex: 1; }
+
 @media (max-width: 640px) {
   .topnav { flex-direction: column; align-items: flex-start; }
   .logout-btn { margin-left: 0; }
   .stats-grid { grid-template-columns: 1fr 1fr; }
   table { font-size: 0.8rem; }
   th, td { padding: 0.4rem; }
+  .catalog-grid { grid-template-columns: 1fr; }
 }