modal: in-app dialog for confirms and import errors

ModalState carries title/message/action as RwSignals; the action is
read at click time so a late confirm wins. Overlay click and Cancel
close; the confirm button runs the stored action. Edition-2024 note:
method calls on non-Copy captures move by value inside nested move
closures, so handlers touch state.open/state.action fields directly.

style/_modal.scss

@@ -0,0 +1,53 @@
+// Modal dialog: dark overlay, centered card, crimson confirm button.
+
+@use 'mixins';
+
+.modal-overlay {
+    position: fixed;
+    inset: 0;
+    z-index: 100;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    background: rgba(0, 0, 0, 0.75);
+}
+
+.modal {
+    max-width: 480px;
+    padding: 24px 32px;
+    background: #1a1a1a;
+    border: 1px solid rgba(255, 255, 255, 0.15);
+    box-shadow: 0 8px 40px rgba(0, 0, 0, 0.8);
+
+    h3 {
+        margin: 0 0 12px;
+        @include mixins.heading;
+        font-weight: 900;
+    }
+
+    p {
+        margin: 0 0 20px;
+        color: rgba(255, 255, 255, 0.75);
+    }
+}
+
+.modal-actions {
+    display: flex;
+    justify-content: flex-end;
+    gap: 12px;
+}
+
+button.modal-cancel {
+    @include mixins.button;
+    background: transparent;
+    border: 1px solid rgba(255, 255, 255, 0.4);
+    color: rgba(255, 255, 255, 0.8);
+    padding: 6px 18px;
+    cursor: pointer;
+}
+
+button.modal-confirm {
+    @include mixins.button;
+    padding: 6px 18px;
+    cursor: pointer;
+}

style/main.scss

@@ -6,6 +6,7 @@
 @use 'sidebar';
 @use 'nav';
 @use 'inventory';
+@use 'modal';
 @use 'vars';