Refactor web build process and update documentation

- Removed Node.js build artifacts from .gitignore and adjusted Makefile to reflect changes in web UI build process, now using server-rendered Go templates instead of React.
- Updated README to clarify the new web UI architecture and output formats, emphasizing the removal of the Node.js build step.
- Added a command to set the number of frames per render task in manager configuration, enhancing user control over rendering settings.
- Improved Gitea workflow by removing unnecessary npm install step, streamlining the CI process.

.gitea/workflows/test-pr.yaml

@@ -12,6 +12,5 @@ jobs:
           go-version-file: 'go.mod'
       - uses: FedericoCarboni/setup-ffmpeg@v3
       - run: go mod tidy
-      - run: cd web && npm install && npm run build
       - run: go build ./...
       - run: go test -race -v -shuffle=on ./...