web: cut over to Vite SPA, retire the Next.js shell (#1444)

Delete the src/app tree, next.config.ts, next-env.d.ts, and the Next-only
guard/header components. Port AppShell/Sidebar/Topbar to react-router and
mount them as a DashboardLayout route over all authenticated routes. Strip
'use client' directives, move globals.css up from the deleted app/ tree,
rewrite tsconfig for Vite/Bundler resolution, drop the next dependency.

Test fixes the port surfaced: jsdom v29 has no window.matchMedia (sidebar
breakpoint) so setup.ts stubs it; the router-boundary specs render inside
ThemeProvider because the chrome's ThemeToggle requires the context.

apps/web/vitest.config.ts

@@ -7,10 +7,6 @@ export default defineConfig({
       '@': fileURLToPath(new URL('./src', import.meta.url)),
     },
   },
-  // tsconfig uses "jsx": "preserve" for Next; tests need esbuild to compile it
-  esbuild: {
-    jsx: 'automatic',
-  },
   test: {
     globals: true,
     environment: 'jsdom',