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>

moves.go

@@ -14,8 +14,7 @@ import (
 // The one exception is a merge: when the source carries a page (index.md) and
 // the destination folder has none, the source's contents can fill the empty
 // container without clobbering anything. That merge only proceeds when merge
-// is true; otherwise the client is told a merge is available so it can ask the
-// user to confirm.
+// is true; otherwise the client is asked to confirm it.
 func (h *handler) handleMove(w http.ResponseWriter, r *http.Request, srcURL, srcFsPath, dstURL string, merge bool) {
 	oldPath := normalizeMovePath(srcURL)
 	if oldPath == "/" {
@@ -51,9 +50,12 @@ func (h *handler) handleMove(w http.ResponseWriter, r *http.Request, srcURL, src
 			return
 		}
 		if !merge {
-			// Ask the client to confirm the merge. The header lets it tell
-			// this apart from an ordinary conflict.
-			w.Header().Set("X-Merge-Available", "1")
+			// Ask the user to confirm: htmx clients get the merge dialog as
+			// the next step; others a conflict they can retry with &merge=1.
+			if isHX(r) {
+				renderDialog(w, "merge", struct{ Dest string }{newPath})
+				return
+			}
 			http.Error(w, "destination already exists — merge folders?", http.StatusConflict)
 			return
 		}
@@ -94,7 +96,7 @@ func (h *handler) handleMove(w http.ResponseWriter, r *http.Request, srcURL, src
 		folderIndexRenameSubtree(strings.TrimPrefix(oldPath, "/"), strings.TrimPrefix(newPath, "/"))
 	}
 
-	http.Redirect(w, r, fileURL(strings.TrimPrefix(newPath, "/"))+"/", http.StatusSeeOther)
+	redirectAfter(w, r, pageURL(newPath))
 }
 
 // canMergeMove reports whether moving onto an existing destination should
@@ -169,7 +171,7 @@ func (h *handler) handleDelete(w http.ResponseWriter, r *http.Request, urlPath,
 		return
 	}
 	folderIndexRemoveSubtree(strings.TrimPrefix(normalizeMovePath(urlPath), "/"))
-	http.Redirect(w, r, parentURL(urlPath), http.StatusSeeOther)
+	redirectAfter(w, r, pageURL(parentURL(urlPath)))
 }
 
 // normalizeMovePath returns the absolute path with any trailing slash removed,