fix: MEDIUM issues #23-#30 — stale deps, export modal, cache race, StrictMode dedup, SeatingService ref, SVG import persist, group-as-block multi-select, session expiry

frontend/src/styles.css

@@ -2212,3 +2212,61 @@ body.drawer-open { overflow: hidden; }
 .tree-drag-after { border-bottom: 2px solid var(--color-primary); }
 .tree-drag-inside { background: var(--color-surface-2); outline: 2px solid var(--color-primary); outline-offset: -2px; }
 .tree-dragging { opacity: 0.5; }
+
+/* Export format selector overlay */
+.export-format-overlay {
+  position: fixed;
+  top: 0; left: 0; right: 0; bottom: 0;
+  background: rgba(0,0,0,0.5);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 9999;
+}
+.export-format-modal {
+  background: var(--color-surface-1, #1e1e2e);
+  border: 1px solid var(--color-border, #333);
+  border-radius: 8px;
+  padding: 20px;
+  min-width: 300px;
+  box-shadow: 0 4px 20px rgba(0,0,0,0.4);
+}
+.export-format-modal h3 {
+  margin: 0 0 16px 0;
+  font-size: 16px;
+  color: var(--color-text, #e0e0e0);
+}
+.export-format-buttons {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
+  margin-bottom: 16px;
+}
+.export-format-btn {
+  padding: 10px 20px;
+  border: 1px solid var(--color-border, #444);
+  border-radius: 4px;
+  background: var(--color-surface-2, #2a2a3e);
+  color: var(--color-text, #e0e0e0);
+  cursor: pointer;
+  font-size: 14px;
+  font-weight: 600;
+  transition: background 0.2s;
+}
+.export-format-btn:hover {
+  background: var(--color-primary, #3498db);
+  color: white;
+}
+.export-format-cancel {
+  width: 100%;
+  padding: 8px;
+  border: 1px solid var(--color-border, #444);
+  border-radius: 4px;
+  background: transparent;
+  color: var(--color-text, #e0e0e0);
+  cursor: pointer;
+  font-size: 13px;
+}
+.export-format-cancel:hover {
+  background: var(--color-surface-2, #2a2a3e);
+}