feat(web): add dockerized web UI with comic library browser

Adds a `yoink serve` command that starts an HTTP server with a
Sonarr/MeTube-inspired dark UI. Features a URL input bar for
triggering downloads, a 150x300 cover grid with filter and sort
controls, a live download queue strip, and toast notifications.

Includes Dockerfile (multi-stage, distroless runtime) and
docker-compose.yml for easy deployment.

cli/healthcheck.go

@@ -0,0 +1,28 @@
+package cli
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var healthcheckCmd = &cobra.Command{
+	Use:    "healthcheck",
+	Short:  "Check if the web server is running (used by Docker HEALTHCHECK)",
+	Args:   cobra.NoArgs,
+	Hidden: true,
+	Run: func(cmd *cobra.Command, args []string) {
+		port, _ := cmd.Flags().GetString("port")
+		resp, err := http.Get(fmt.Sprintf("http://localhost:%s/health", port))
+		if err != nil || resp.StatusCode != http.StatusOK {
+			os.Exit(1)
+		}
+	},
+}
+
+func init() {
+	healthcheckCmd.Flags().StringP("port", "p", "8080", "Port the server is listening on")
+	cli.AddCommand(healthcheckCmd)
+}