feat: add application preloader, identity roles, and resilient database initialization with automated seeding

src/NexusReader.UI.Shared/wwwroot/app.css

@@ -99,4 +99,57 @@ h1:focus {
     color: white;
     margin: 1rem;
     border-radius: 8px;
-}
+}
+
+/* Preloader Styles */
+#app-preloader, .app-preloader {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    background: radial-gradient(circle at center, #1a1a1a 0%, var(--nexus-bg) 100%);
+    display: flex;
+    flex-direction: column;
+    justify-content: center;
+    align-items: center;
+    z-index: 9999;
+    transition: opacity 0.8s ease, visibility 0.8s;
+}
+
+#app-preloader.loaded {
+    opacity: 0;
+    visibility: hidden;
+}
+
+.preloader-spinner {
+    width: 80px;
+    height: 80px;
+    border: 3px solid rgba(0, 255, 153, 0.1);
+    border-top: 3px solid var(--nexus-neon);
+    border-radius: 50%;
+    animation: spin 1s cubic-bezier(0.4, 0, 0.2, 1) infinite;
+    filter: drop-shadow(0 0 10px var(--nexus-neon));
+    margin-bottom: 20px;
+}
+
+.preloader-text {
+    color: var(--nexus-neon);
+    font-family: var(--nexus-font-sans);
+    letter-spacing: 4px;
+    text-transform: uppercase;
+    font-size: 0.8rem;
+    font-weight: 500;
+    animation: pulse 2s infinite;
+}
+
+@keyframes spin {
+    0% { transform: rotate(0deg); }
+    100% { transform: rotate(360deg); }
+}
+
+@keyframes pulse {
+    0%, 100% { opacity: 1; transform: scale(1); }
+    50% { opacity: 0.5; transform: scale(0.95); }
+}
+