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/package.json

@@ -3,22 +3,19 @@
   "version": "0.0.2",
   "private": true,
   "scripts": {
-    "build": "node ../../scripts/build-web.mjs",
-    "build:vite": "vite build",
-    "dev": "next dev -p 3101",
-    "dev:vite": "vite",
+    "build": "vite build",
+    "dev": "vite",
+    "preview": "vite preview",
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests",
-    "test:e2e": "playwright test",
-    "start": "next start -p 3101"
+    "test:e2e": "playwright test"
   },
   "dependencies": {
     "@mosaicstack/design-tokens": "workspace:^",
     "@mosaicstack/types": "workspace:^",
     "better-auth": "^1.5.5",
     "clsx": "^2.1.0",
-    "next": "^16.0.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-markdown": "^10.1.0",