settings: make the user and audit-log dialogs opaque

The Add User dialog was see-through over its own overlay in dark mode. Both it
and the audit-log detail dialog set background: var(--bg-card), which is
translucent in dark mode - frontend/CLAUDE.md already names this trap and says
a modal panel must use var(--bg-card-solid). The shared Modal.vue and the
.modal rule in style.css were already correct; these two hand-rolled panels
were not.

Audited every modal-ish panel in the app while here. The remaining
var(--bg-card) uses are correct and deliberate: .relationships-section and
.permission-category are cards, and the audit-log .filters rules are form
controls - translucency is the intended look on all of them.

One more was worth changing: the warranty asset-search dropdown floats above
the form INSIDE a modal, so a translucent panel let the fields show through the
suggestions. That one is now solid too.

frontend/src/views/settings/AuditLogs.vue

@@ -319,7 +319,8 @@ onMounted(loadLogs)
 }
 
 .modal {
-  background: var(--bg-card);
+  /* SOLID, not var(--bg-card) - translucent in dark mode over the overlay. */
+  background: var(--bg-card-solid);
   border: 1px solid var(--border);
   border-radius: 8px;
   min-width: 500px;

frontend/src/views/settings/UsersList.vue

@@ -636,7 +636,9 @@ onMounted(loadData)
 }
 
 .modal {
-  background: var(--bg-card);
+  /* SOLID, not var(--bg-card): the card variable is translucent in dark mode,
+     which left this dialog see-through over its own overlay. */
+  background: var(--bg-card-solid);
   border: 1px solid var(--border);
   border-radius: 8px;
   width: 500px;