feat(web): add Shell component with header, sidebar, and user menu

Persistent chrome used by every authenticated route. Sidebar hides
below md:; mobile nav is a separate concern for a later plan.

web/vitest.config.ts

@@ -1,8 +1,9 @@
 import { sveltekit } from '@sveltejs/kit/vite';
+import { svelteTesting } from '@testing-library/svelte/vite';
 import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
-  plugins: [sveltekit()],
+  plugins: [sveltekit(), svelteTesting()],
   test: {
     environment: 'jsdom',
     include: ['src/**/*.test.ts'],