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/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)