search: integrate find/replace into search overlay, add toolbar buttons

Moves board-wide find/replace from standalone overlay into the search
panel as a toggleable replace row. Adds editor toolbar find/replace
button. Ctrl+Shift+H now opens search in replace mode.

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

public/js/modules/editor.js

@@ -1001,7 +1001,7 @@ const replRow    = $('replace-row');
 let findMatches = [];
 let findCurrent = -1;
 
-function openFindBar(withReplace = true) {
+export function openFindBar(withReplace = true) {
     if (!findBar) return;
     findBar.classList.remove('hidden');
     replRow?.classList.toggle('hidden', !withReplace);