feat: add modlist export to Arma 3 Launcher HTML format

- RenderModlistHTML: generates compatible preset HTML from a modlist
- GET /api/modlists/:id/export: returns downloadable .html file
- Export button on ModlistEditor page
- Skips local mods (no workshop ID), includes only Steam workshop mods

backend/internal/api/modlists.go

@@ -1,7 +1,9 @@
 package api
 
 import (
+	"fmt"
 	"net/http"
+	"strings"
 
 	"arma3-web-server/internal/models"
 	"arma3-web-server/internal/services"
@@ -121,6 +123,25 @@ func (h *Handler) ImportModlist(c *gin.Context) {
 	c.JSON(http.StatusCreated, ml)
 }
 
+func (h *Handler) ExportModlist(c *gin.Context) {
+	ml, err := h.modlists.Get(c.Param("id"))
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": err.Error()})
+		return
+	}
+
+	html, err := services.RenderModlistHTML(ml.Name, ml.Mods)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+		return
+	}
+
+	filename := fmt.Sprintf("%s.html", strings.ReplaceAll(ml.Name, `"`, `_`))
+	c.Header("Content-Type", "text/html; charset=utf-8")
+	c.Header("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"`, filename))
+	c.String(http.StatusOK, html)
+}
+
 type modStatus struct {
 	ID         string `json:"id"`
 	Name       string `json:"name"`

backend/internal/api/router.go

@@ -80,6 +80,7 @@ func (h *Handler) SetupRoutes(r *gin.Engine) {
 		api.GET("/modlists/:id/check", h.CheckModlistMods)
 		api.POST("/modlists/:id/download-missing", h.DownloadMissingMods)
 		api.POST("/modlists/:id/update-all", h.UpdateAllMods)
+		api.GET("/modlists/:id/export", h.ExportModlist)
 
 		api.GET("/mods", h.ListMods)
 		api.DELETE("/mods", h.DeleteMod)