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.

internal/manager/renderer_test.go

@@ -0,0 +1,13 @@
+package api
+
+import "testing"
+
+func TestNewUIRendererParsesTemplates(t *testing.T) {
+	renderer, err := newUIRenderer()
+	if err != nil {
+		t.Fatalf("newUIRenderer returned error: %v", err)
+	}
+	if renderer == nil || renderer.templates == nil {
+		t.Fatalf("renderer/templates should not be nil")
+	}
+}