feat: migrate to Next.js for Mini App and enhance project structure

- Replaced the previous webapp with a new Mini App built using Next.js, improving performance and maintainability.
- Updated the `.gitignore` to exclude Next.js build artifacts and node modules.
- Revised documentation in `AGENTS.md`, `README.md`, and `architecture.md` to reflect the new Mini App structure and technology stack.
- Enhanced Dockerfile to support the new build process for the Next.js application.
- Updated CI workflow to build and test the Next.js application.
- Added new configuration options for the Mini App, including `MINI_APP_SHORT_NAME` for improved deep linking.
- Refactored frontend testing setup to accommodate the new structure and testing framework.
- Removed legacy webapp files and dependencies to streamline the project.

.gitea/workflows/ci.yml

@@ -45,8 +45,6 @@ jobs:
         with:
           node-version: "20"
 
-      - name: Webapp tests
+      - name: Webapp (Next.js) build and test
         run: |
-          cd webapp
-          npm ci
-          npm run test
+          cd webapp-next && npm ci && npm test && npm run build