Add custom short code support to web UI

Allow users to optionally specify their own short code when
shortening a URL, instead of always generating a random one.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

handler.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"embed"
 	"net/http"
+	"regexp"
 	"strings"
 
 	"git.wntrmute.dev/mc/mcdsl/auth"
@@ -82,6 +83,14 @@ func (srv *server) postURL(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	customShort := strings.TrimSpace(r.FormValue("custom"))
+	if customShort != "" {
+		if !regexp.MustCompile(`^[a-zA-Z0-9]+$`).MatchString(customShort) {
+			http.Error(w, "custom short code must be letters and numbers only", http.StatusBadRequest)
+			return
+		}
+	}
+
 	url, err = links.CleanString(url, stripQuery)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
@@ -89,7 +98,7 @@ func (srv *server) postURL(w http.ResponseWriter, r *http.Request) {
 	}
 
 	ctx := context.Background()
-	short, err := links.StoreURL(ctx, srv.db, url)
+	short, err := links.StoreURL(ctx, srv.db, url, customShort)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return