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>

assets/layout.html

@@ -8,6 +8,8 @@
         <link rel="preload" href="/_/fonts/IosevkaEtoile.woff2" as="font" type="font/woff2" crossorigin />
         <link rel="preload" href="/_/fonts/IosevkaSlab.woff2" as="font" type="font/woff2" crossorigin />
         <link rel="stylesheet" href="{{asset "style.css"}}" />
+        <script src="{{asset "vendor/htmx.min.js"}}" defer></script>
+        <script src="{{asset "hx.js"}}" defer></script>
         <script src="{{asset "modal.js"}}"></script>
         <script src="{{asset "global-shortcuts.js"}}"></script>
         <script src="{{asset "history-nav.js"}}"></script>
@@ -77,6 +79,7 @@
             {{if not .EditMode}}{{block "aside" .}}{{end}}{{end}}
         </div>
         {{block "extras" .}}{{end}}
+        <div id="modal-host"></div>
     </body>
 </html>
 {{end}}