WIP df2w: an open empty panel is Some(vec![]), closed is None

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

web/app.js

@@ -594,12 +594,12 @@ function edit(td, row, col, seed) {
   });
 }
 
-/** A side panel. An empty row list is what says it is closed — the page asks
- *  no question about the mode, exactly as `paintHelp` does not. */
+/** A side panel. `null` is what says it is closed, and `[]` is open and empty:
+ *  two facts, never inferred from a count (improvise-df2w). */
 function paintPanel(host, rows, build) {
-  host.hidden = rows.length === 0;
+  host.hidden = rows === null;
   const list = document.createElement("ul");
-  rows.forEach((row, index) => {
+  (rows ?? []).forEach((row, index) => {
     const li = build(row);
     li.dataset.index = String(index);
     list.appendChild(li);