fix: validate owner/repo split in create_issue.html before submission

Add client-side validation to ensure a repository is selected before
form submission. Split owner/repo on both change and submit events.
Show inline error messages via form-error div. Update CreateIssue
handler to return HTMX-friendly HTML error fragments on 400/500.

Closes leeworks-agents/gitea-mobile#30

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

internal/handlers/handlers.go

@@ -493,6 +493,12 @@ func (h *Handler) CreateIssue(w http.ResponseWriter, r *http.Request) {
 	body := r.FormValue("body")
 
 	if owner == "" || repo == "" || title == "" {
+		if isHTMX(r) {
+			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			w.WriteHeader(http.StatusBadRequest)
+			fmt.Fprint(w, `<span class="empty">owner, repo, and title are required</span>`)
+			return
+		}
 		http.Error(w, "owner, repo, and title are required", http.StatusBadRequest)
 		return
 	}
@@ -500,6 +506,12 @@ func (h *Handler) CreateIssue(w http.ResponseWriter, r *http.Request) {
 	issue, err := h.Client.CreateIssue(r.Context(), token, owner, repo, title, body, nil)
 	if err != nil {
 		slog.Error("failed to create issue", "error", err)
+		if isHTMX(r) {
+			w.Header().Set("Content-Type", "text/html; charset=utf-8")
+			w.WriteHeader(http.StatusInternalServerError)
+			fmt.Fprint(w, `<span class="empty">Failed to create issue. Please try again.</span>`)
+			return
+		}
 		http.Error(w, "failed to create issue", http.StatusInternalServerError)
 		return
 	}