Layouts keep every tile they are given, and show what is on offer

The bug behind this: for seventeen days the server rebuilt each saved tile
from the fields it knew and dropped the rest — including `variant`, so a
layout of five different timers came back from the server as five "All
timers". The client had carried `variant` since multi-tile modules arrived;
the server was the one that lagged, and it is the part of ProdDash that
always will (a booth updates weeks after the dev machine). So the server
stops deciding which tile fields exist: it checks the shape of the ones it
knows, keeps any other small plain-JSON field a tile carries, drops only
retired ones, stamps the file `v: 2`, and refuses a layout over a hundred
tiles instead of quietly saving the first hundred. A test round-trips a
fully-populated tile plus one the server has never heard of, and fails if
anything is lost.

Then the rule the arrangement is loaded by. A layout keeps every tile it is
given; the board shows the ones whose module is installed and enabled and —
for a tile added from a picker entry — whose entry is offered right now. The
rest keep their place and draw nothing, and appear the moment the registry
offers them again: a Service timer waits for PCO Plan, a transcript for its
channel. Never the module's default in their place. Because a module's entry
list can change with no event to mark it (PCO Plan comes back and, seconds
later, the timers module is offering its timers again), the registry is
re-read every few seconds while anything is folded away, and not at all
otherwise. The timers module's placeholder card for a missing solo timer
goes with this: nothing is drawn, the dot says why.

Loading says what it did — "Loaded 'FOH': 9 tiles, 2 not shown — Service,
#ops aren't offered right now" — with Undo in the message. Saving over a
name asks first, and the server keeps the copy it replaced one back; Admin →
Saved layouts gets Restore previous, which swaps the two so a restore is one
restore from undone. The browser keeps one good copy of its own layout
behind the live one and falls back to it when the live one will not parse.

Shell 1.8.0, timers 1.4.1. The layouts already saved on the booth machines
had their variants stripped before this and have to be saved again from a
browser that still holds the right tiles — nothing here recovers what the
files no longer contain.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

public/admin/admin.js

@@ -909,7 +909,25 @@ async function loadLayouts() {
         else toast(e.message, true);
       }
     });
-    row.append(name, meta, rename, del);
+    // The copy the last save replaced, one back. Restoring swaps the two,
+    // so a restore is itself one restore away from being undone.
+    const restore = document.createElement('button');
+    restore.className = 'btn';
+    restore.textContent = 'Restore previous';
+    restore.title = 'Put back the copy the last save replaced';
+    restore.hidden = !layout.hasPrevious;
+    restore.addEventListener('click', async () => {
+      if (!confirm(`Go back to the previous version of “${layout.name}”? The current one becomes its previous version, so this can be undone the same way.`)) return;
+      try {
+        await api(`/api/layouts/${encodeURIComponent(layout.name)}/restore`, { method: 'POST', body: '{}' });
+        toast('Previous version restored');
+        loadLayouts();
+      } catch (e) {
+        if (e.authRequired) showLogin();
+        else toast(e.message, true);
+      }
+    });
+    row.append(name, meta, restore, rename, del);
     layoutsList.appendChild(row);
   }
 }