fix(ui): harden modal lifecycle and keychain access

src/index.css

@@ -604,9 +604,11 @@ html[data-list-density="relaxed"] {
   .app-modal-backdrop {
     /* Keep the scrim animation from applying opacity to the modal subtree. */
     background-color: hsl(0 0% 0% / 0.2);
-    animation: modal-backdrop-in 150ms ease-out;
+    animation: modal-backdrop-in 167ms cubic-bezier(0, 0, 0, 1);
     overflow: auto;
+    overscroll-behavior: contain;
     padding: 16px;
+    scrollbar-gutter: stable;
   }
 
   .app-shell--window-chrome .app-modal-backdrop {
@@ -623,12 +625,21 @@ html[data-list-density="relaxed"] {
 
   .app-modal {
     background: hsl(var(--surface-overlay));
-    backdrop-filter: blur(40px);
-    -webkit-backdrop-filter: blur(40px);
     border: 1px solid hsl(var(--border-modal));
     border-radius: 12px;
     box-shadow: 0 20px 40px hsl(var(--shadow-color));
-    animation: modal-in 150ms ease-out;
+    animation: modal-in 167ms cubic-bezier(0.2, 0.8, 0.2, 1);
+    contain: layout;
+  }
+
+  .keychain-modal {
+    max-width: 560px;
+    max-height: min(680px, 100%);
+  }
+
+  .properties-modal {
+    max-width: 100%;
+    max-height: 100%;
   }
 
   /* Add Download window */
@@ -641,8 +652,6 @@ html[data-list-density="relaxed"] {
     --add-highlight: hsl(0 0% 100% / 0.08);
     background:
       linear-gradient(180deg, hsl(var(--surface-overlay) / 0.94), hsl(var(--bg-modal) / 0.98));
-    backdrop-filter: blur(22px) saturate(1.12);
-    -webkit-backdrop-filter: blur(22px) saturate(1.12);
     border-color: hsl(var(--border-modal));
     border-radius: 14px;
     box-shadow:
@@ -3350,7 +3359,7 @@ html[dir="rtl"] .download-context-menu-chevron {
 }
 
 @keyframes modal-in {
-  from { opacity: 0; transform: scale(0.98); }
+  from { opacity: 0; transform: translateY(4px) scale(0.99); }
   to { opacity: 1; transform: scale(1); }
 }
 
@@ -3376,6 +3385,13 @@ html[dir="rtl"] .download-context-menu-chevron {
 }
 
 @media (prefers-reduced-motion: reduce) {
+  .app-modal-backdrop,
+  .app-modal {
+    animation: none !important;
+    transform: none !important;
+    opacity: 1 !important;
+  }
+
   *,
   *::before,
   *::after {