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/pages/snippet/SnippetsPage.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { useTranslation } from 'react-i18next';
-import { Code, Pencil, Trash2, Plus, Download, Braces } from 'lucide-react';
+import { Settings, Pencil, Trash2, Plus, Download, Braces } from 'lucide-react';
 import { parse as yamlParse } from 'yaml';
 import { snippets } from '../../api';
 import type { Snippet } from '../../api';
@@ -20,7 +20,7 @@ function SnippetCard({ snippet, onDelete }: { snippet: Snippet; onDelete: (id: s
           align="right"
           trigger={
             <Button variant="ghost" size="sm" aria-label={t('snippets.menu_label')}>
-              <Code size={14} />
+              <Settings size={14} />
             </Button>
           }
           items={[