Migrate Next.js to Vite (#1397)

Vite is much simpler to use than Next.js and we don't need any of the
features Next has that Vite doesn't have.

Benefits of moving to Vite are:
- Much better performance in dev and prod environments
- Much better build times
- Actual support for static exports, no vendor lock-in of having to use
Vercel
- Support for runtime environment variables/loading config from `.env`
files
- No annoying backwards-incompatible changes on major releases of Next
- Better i18n support without having to define getServerSideProps on
every page
- Better bundle optimization
- No opt-out Vercel telemetry 

Also replaces yarn by pnpm and upgrades mantine to 8.3

.github/workflows/frontend.yml

@@ -19,17 +19,20 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
-      - name: Setup yarn
+      - name: Enable Corepack
+        run: corepack enable
+
+      - name: Setup pnpm
         uses: actions/setup-node@v6
         with:
           node-version: '22'
-          cache: 'yarn'
-          cache-dependency-path: frontend/yarn.lock
+          cache: 'pnpm'
+          cache-dependency-path: frontend/pnpm-lock.yaml
 
       - name: Install npm modules
-        run: yarn
+        run: pnpm i
         working-directory: frontend
 
       - name: Run prettier, lint, jest checks
-        run: yarn test
+        run: pnpm test
         working-directory: frontend