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/css/style.css

@@ -907,6 +907,7 @@ body.anim-slow .board-card.card-pulse-local {
 .search-hit { scroll-margin: 8px; }
 .search-hit.active { background: var(--t-raised); }
 .search-hit mark, #search-preview-content mark { background: none; color: var(--t-accent); font-weight: 700; }
+#search-replace-toggle.active { border-color: var(--t-accent); color: var(--t-accent); }
 
 /* Context menu */
 #ctx-menu .ctx-item, #bucket-ctx .ctx-item {