feat(app): add environment import/export and sidebar footer info

- add environment import/export API endpoints and client integration

- add environment export actions and toolbar import/export buttons in UI

- move theme switcher to sidebar bottom and show root package version

- switch snippet card menu icon to cog for consistent options affordance

client/src/App.module.css

@@ -30,6 +30,28 @@
   padding: var(--space-2) 0;
 }
 
+.sideContent {
+  display: flex;
+  flex-direction: column;
+  min-height: 100%;
+}
+
+.sideFooter {
+  margin-top: auto;
+  padding: var(--space-3) var(--space-2) var(--space-2);
+  border-top: var(--border-width) solid var(--color-border);
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--space-2);
+}
+
+.version {
+  color: var(--color-text-muted);
+  font-size: var(--font-size-xs);
+  white-space: nowrap;
+}
+
 .navItem {
   display: flex;
   align-items: center;