Serve delete, move and merge dialogs as htmx fragments

The server renders the dialogs (?dialog=delete|move) and answers a move
that needs confirmation with the merge dialog itself, so the client no
longer builds action URLs or interprets X-Merge-Available. Successful
mutations send HX-Redirect, which hx.js turns into a history replace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dialogs.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"html/template"
+	"log"
+	"net/http"
+	"path"
+	"strings"
+)
+
+var dialogTmpl = template.Must(template.New("dialogs").ParseFS(assets, "assets/page/dialogs.html"))
+
+// isHX reports whether r was issued by htmx.
+func isHX(r *http.Request) bool {
+	return r.Header.Get("HX-Request") == "true"
+}
+
+// redirectAfter sends the client to target once a mutation succeeded. htmx
+// requests get HX-Redirect (assets/hx.js turns it into a history replace, so
+// Back skips the pre-mutation page); plain form posts get a 303.
+func redirectAfter(w http.ResponseWriter, r *http.Request, target string) {
+	if isHX(r) {
+		w.Header().Set("HX-Redirect", target)
+		w.WriteHeader(http.StatusNoContent)
+		return
+	}
+	http.Redirect(w, r, target, http.StatusSeeOther)
+}
+
+// pageURL returns the escaped, slash-terminated browser URL of the page at
+// the wiki path p ("/" or "/a/b", trailing slash optional).
+func pageURL(p string) string {
+	p = strings.Trim(p, "/")
+	if p == "" {
+		return "/"
+	}
+	return fileURL(p) + "/"
+}
+
+// renderDialog writes the named dialog from dialogs.html.
+func renderDialog(w http.ResponseWriter, name string, data any) {
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	if err := dialogTmpl.ExecuteTemplate(w, name, data); err != nil {
+		log.Printf("dialog template error: %v", err)
+	}
+}
+
+// serveDialog handles GET ?dialog=<name> for the page at urlPath.
+func (h *handler) serveDialog(w http.ResponseWriter, r *http.Request, urlPath string) {
+	page := normalizeMovePath(urlPath)
+	if page == "/" {
+		http.Error(w, "not available for the wiki root", http.StatusBadRequest)
+		return
+	}
+	switch r.URL.Query().Get("dialog") {
+	case "delete":
+		renderDialog(w, "delete", struct{ Path string }{urlPath})
+	case "move":
+		// parent comes from the folder picker; default to the current one.
+		parent := r.URL.Query().Get("parent")
+		if parent == "" {
+			parent = path.Dir(page)
+		}
+		renderDialog(w, "move", struct{ Parent, Name string }{parent, path.Base(page)})
+	default:
+		http.NotFound(w, r)
+	}
+}