feat(10-04): collapsible panel with flyout, user menu with logout, breadcrumbs and dark mode

- useSidebar: collapsed below 1100px (matchMedia) or by the admin's choice,
  persisted as a boolean under summer-admin.sidebar; the viewport never
  overwrites the stored choice
- SectionPanel collapse and rail expand buttons; SectionFlyout (role menu)
  opens on hover, focus, Enter or ArrowDown on a rail item, closes on Esc or
  about 200 ms after leaving and returns focus to the rail item
- Breadcrumbs with plugin, controller and record crumbs; UserMenu (Reka
  DropdownMenu) with initials, name, role and Wyloguj
- useAuth.logout POSTs /auth/logout, clears user, navigation and settings
  and routes to login even when the call fails
- applyColorScheme toggles .dark from prefers-color-scheme (no toggle, A6)
- newest toast first, 200 ms fade and scale for dialogs and toasts
- one relation-manager type constant in the registry
- shell smoke tests; tests default to a desktop, light matchMedia

admin/src/styles/main.css

@@ -104,6 +104,7 @@
 }
 
 .dark {
+  color-scheme: dark;
   --c-bg: #111726;
   --c-surface: #182033;
   --c-subtle: #1f283d;
@@ -164,3 +165,56 @@
     color: var(--c-placeholder);
   }
 }
+
+/* Motion (design: Interactions, Transitions): subtle only. Hover and
+   background changes use the 150 ms ease-out utilities on each control;
+   dialogs and their backdrop fade in and scale from 0.98 over 200 ms; toasts
+   fade in the same way. Closing is immediate, so Reka's presence never waits
+   on an exit animation. No decorative animation. */
+@keyframes summer-fade-in {
+  from {
+    opacity: 0;
+  }
+}
+
+@keyframes summer-dialog-in {
+  from {
+    opacity: 0;
+    scale: 0.98;
+  }
+}
+
+@layer components {
+  /* Backdrops of the picker (data-dialog-overlay) and of ConfirmDialog. */
+  .bg-overlay[data-state="open"] {
+    animation: summer-fade-in 200ms ease-out;
+  }
+
+  [data-dialog-content][data-state="open"],
+  [role="alertdialog"][data-state="open"] {
+    animation: summer-dialog-in 200ms ease-out;
+  }
+
+  .toast-enter-active {
+    transition:
+      opacity 200ms ease-out,
+      scale 200ms ease-out;
+  }
+
+  .toast-enter-from {
+    opacity: 0;
+    scale: 0.98;
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .bg-overlay,
+  [data-dialog-content],
+  [role="alertdialog"] {
+    animation: none !important;
+  }
+
+  .toast-enter-active {
+    transition: none;
+  }
+}