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/declarations.d.ts

@@ -6,3 +6,5 @@ declare module '*.css' {
 interface ImportMetaEnv {
   readonly VITE_API_URL?: string;
 }
+
+declare const __APP_VERSION__: string;