feat: enhance app layout with new shell and navigation styles

src/ASTRAIN.Client/wwwroot/css/app.css

@@ -205,6 +205,118 @@ p {
     gap: 1rem;
 }
 
+.app-shell {
+    min-height: 100vh;
+    background: #0b0b0b;
+    color: #f5f5f5;
+    display: flex;
+    flex-direction: column;
+}
+
+.main-content {
+    flex: 1;
+    padding: 1.5rem 1.25rem 6rem;
+}
+
+.brand-header {
+    text-align: center;
+    font-weight: 800;
+    font-size: 1.1rem;
+    padding: 1.25rem 0 0.25rem;
+}
+
+.brand-white {
+    color: #ffffff;
+}
+
+.brand-red {
+    color: #ff3b3b;
+}
+
+.bottom-nav {
+    position: fixed;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    height: 96px;
+    background: #141414;
+    border-top: 1px solid #2a2a2a;
+    display: grid;
+    grid-template-columns: 1fr auto 1fr;
+    align-items: center;
+    padding: 0 1.5rem;
+    gap: 1rem;
+    z-index: 10;
+}
+
+.nav-item {
+    color: #bdbdbd;
+    text-decoration: none;
+    font-weight: 700;
+    font-size: 0.9rem;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    height: 44px;
+    border: 1px solid transparent;
+    letter-spacing: 0.08em;
+    text-transform: uppercase;
+    width: 100%;
+    max-width: 160px;
+    margin: 0 auto;
+    justify-self: center;
+    text-align: center;
+    transition: color 0.2s ease, border-color 0.2s ease, background 0.2s ease;
+}
+
+.nav-item.active {
+    color: #ffffff;
+    font-weight: 700;
+    border-color: #ff3b3b;
+    box-shadow: 0 0 0 2px rgba(255, 59, 59, 0.15);
+    background: rgba(255, 59, 59, 0.12);
+}
+
+.nav-item:hover {
+    color: #ffffff;
+    background: rgba(255, 255, 255, 0.06);
+}
+
+.nav-logo {
+    width: 80px;
+    height: 80px;
+    color: #ffffff;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-weight: 700;
+    border-radius: 999px;
+    font-size: 0.7rem;
+    letter-spacing: 0.1em;
+}
+
+.nav-logo img {
+    width: 80px;
+    height: 80px;
+    border-radius: 10px;
+    object-fit: cover;
+}
+
+@media (min-width: 768px) {
+    .main-content {
+        padding: 2rem 2.5rem 7rem;
+        max-width: 960px;
+        margin: 0 auto;
+    }
+
+    .bottom-nav {
+        left: 50%;
+        transform: translateX(-50%);
+        max-width: 720px;
+        border-radius: 24px 24px 0 0;
+    }
+}
+
 .loading-screen img {
     width: 96px;
     height: 96px;