fix(ui): polish editor layouts and confirm scene delete

Stretch key action buttons, center empty audio/image pickers, and require confirmation before deleting a scene.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/renderer/editor/MaterialsModals.module.css

@@ -74,6 +74,24 @@
   overflow: hidden;
 }
 
+.sideAddBtn {
+  width: 100%;
+  flex: 0 0 auto;
+}
+
+.sideAddBtn > * {
+  width: 100%;
+  max-width: 100%;
+  display: flex;
+  box-sizing: border-box;
+}
+
+.sideAddBtn button {
+  width: 100%;
+  box-sizing: border-box;
+  flex: 1;
+}
+
 .list {
   display: grid;
   gap: 8px;
@@ -245,6 +263,21 @@
   position: relative;
 }
 
+.imageDropEmpty {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 10px;
+  min-height: 112px;
+  text-align: center;
+  padding: 8px 4px;
+}
+
+.imageDropEmpty > * {
+  max-width: 100%;
+}
+
 .imageDropOver {
   border-color: var(--color-accent, #a78bfa);
 }