feat(ui): replace text loading states with skeleton loaders

Add skeleton CSS classes to main.css for consistent loading UI
Apply skeleton loading states to Admin, Habitat, Item, Pokemon, Recipe, and Auth views
Improve perceived performance and accessibility during data fetching

frontend/src/styles/main.css

@@ -728,6 +728,50 @@ button:disabled,
   height: 28px;
 }
 
+.page-header--skeleton {
+  pointer-events: none;
+}
+
+.skeleton-detail-section {
+  pointer-events: none;
+}
+
+.skeleton-detail-section .detail-section__body {
+  gap: 14px;
+}
+
+.skeleton-row-list li {
+  min-height: 43px;
+}
+
+.skeleton-appearance-row {
+  display: grid;
+  grid-template-columns: max-content minmax(0, 1fr);
+  gap: 12px;
+}
+
+.skeleton-summary {
+  display: grid;
+  gap: 6px;
+  width: 100%;
+}
+
+.skeleton-summary div {
+  display: grid;
+  grid-template-columns: 72px minmax(0, 1fr);
+  gap: 8px;
+}
+
+.skeleton-form-stack {
+  display: grid;
+  gap: 14px;
+}
+
+.skeleton-auth-state {
+  display: grid;
+  gap: 12px;
+}
+
 @keyframes shimmer {
   to {
     background-position: -200% 0;