feat: select text across bullets and delete it like a text editor; Ctrl+Z in tasks; notes and lists without emoji; custom done columns complete tasks

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

frontend/src/index.css

@@ -606,6 +606,11 @@ body {
   }
 }
 
+/* Selection over several bullets in a note (OutlinerTree, CSS Highlight API) */
+::highlight(outline-range) {
+  background-color: color-mix(in srgb, var(--accent-primary) 30%, transparent);
+}
+
 /* Read-only sections (e.g. server settings in the demo): everything inside looks switched off */
 fieldset:disabled :is(button, input, select, textarea) {
   opacity: 0.5;