feat(upload): add pause and cancel controls for active uploads

- Add CSS grid layout for upload-active-actions and hidden state
- Implement JavaScript logic for pausing and cancelling uploads with confirmation
- Add test to verify home page includes upload control elements

backend/libs/handlers/static_test.go

@@ -6,6 +6,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -29,6 +30,30 @@ func TestSetStaticCacheHeaders(t *testing.T) {
 	}
 }
 
+func TestHomeIncludesActiveUploadControls(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+
+	request := httptest.NewRequest(http.MethodGet, "/", nil)
+	response := httptest.NewRecorder()
+	app.Home(response, request)
+
+	if response.Code != http.StatusOK {
+		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
+	}
+	for _, want := range []string{
+		`id="upload-active-actions"`,
+		`id="cancel-upload"`,
+		`id="pause-upload"`,
+		`Cancel Upload`,
+		`Pause Upload`,
+	} {
+		if !strings.Contains(response.Body.String(), want) {
+			t.Fatalf("home page missing %q", want)
+		}
+	}
+}
+
 func TestWebManifestIncludesShareTarget(t *testing.T) {
 	data, err := os.ReadFile(filepath.Join("..", "..", "static", "site.webmanifest"))
 	if err != nil {