docs: manual test scenarios for the concurrent-write / editor bugs

Covers the four classes found this session (stale editor payload, editor
dismissed without the discard gate, missing withEncryptedMeta, view-blind
UI) with the surrounding cases, not just the ones reported.

Also fixes one more instance found while auditing: deleting a folder
reassigns its entries to All server-side, but an editor open on one of them
kept pointing at the deleted folder and re-created it on Save.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.js

@@ -5526,6 +5526,11 @@ async function deleteFolder(name, entryCount) {
             headers: authHeaders(),
         });
         if (state.view === 'folder:' + name) state.view = 'all';
+        // The server reassigned the folder's entries to All. An editor left
+        // open on one of them still points at the deleted folder and would
+        // re-create it on Save (its <option> is gone, so the select reads '').
+        if (soState && soState.original.folder === name)
+            editorAbsorbChange(soState.id, { folder: 'All' });
         await loadFolders();
         await loadEntries();
         render();