Let guests try a demo project without an account (#428)

The editor was only open to somebody with an account. A guest now picks
a demo project on the sign-in screen and gets the whole editor on a copy
held in the tab. The server writes nothing for a guest, so there is no
guest session, no guest folder and no quota to keep.

- Demo copies are frozen copies like a release, in data/demos/. An
  administrator makes, updates and removes one with "Demo copy..." in
  the project's main menu. GET demos and GET demos/{token} need no
  session. An empty project is just a demo that is empty.
- Demo material: an administrator offers an uploaded file with "Offer
  as demo" in the pickers (PATCH {demo}, data/demo-files.json). Every
  list carries it with demo: true, for accounts and guests alike.
  Signed out, a list holds the demo material alone, without the owner.
- An offered file is only an administrator's to delete. A file a demo
  copy names counts as used, like one a release names.
- The guest's project lives in MemoryProjectStore. Every store that
  would write refuses in the tab before a request is made, and the
  autosave is not attached at all.
- The main menu drops projects, snapshots and release for a guest. The
  pickers drop Upload and Delete, a dropped or pasted file is refused,
  and a recording is saved as a download.
- The toolbar says the work is not saved. The Account menu holds
  "Sign in..." instead of Logout and the password.
- A guest's demo never goes into the address: a reload brings the
  sign-in screen back, not the copy. Found by e2e 70.
- GET plugins needs no session any more, since a demo may switch one
  on. It only reads the server's own modules.

Tests:
- e2e 70 (atx): ALL PASSED, 13 checks. Counter-test with the guest
  autosave sent to the server: 70f and 70g red on the PUT.
- tests/core/guest.test.ts: 9 tests. PHP: demos 20, files 46,
  rights 80 checks (the matrix asks every new route).
- Gate (atx): CHECK EXIT 0, 314 files, 4852 tests.

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

api/lib/ReleaseStore.php

@@ -13,6 +13,9 @@ declare(strict_types=1);
  *
  * Like the snapshots, the store checks nothing about who asks: the router reaches the project half
  * only after `Store::load` found the project for that user.
+ *
+ * Demo copies (#428) are the same thing on `data/demos/`: a frozen copy a guest opens in the editor
+ * instead of the player, made and updated by an administrator.
  */
 final class ReleaseStore
 {
@@ -82,6 +85,25 @@ final class ReleaseStore
         ];
     }
 
+    /**
+     * Every copy there is, by name: what a guest picks a demo from (#428), which is this store on a
+     * directory of its own.
+     *
+     * @return list<array{token:string,name:string,publishedAt:string}>
+     */
+    public function list(): array
+    {
+        $items = [];
+        foreach (glob($this->dir . '/*.json') ?: [] as $path) {
+            $release = $this->load(basename($path, '.json'));
+            if ($release !== null) {
+                $items[] = ['token' => $release['token'], 'name' => $release['name'], 'publishedAt' => $release['publishedAt']];
+            }
+        }
+        usort($items, static fn(array $a, array $b): int => strnatcasecmp($a['name'], $b['name']));
+        return $items;
+    }
+
     /** Makes the link dead. False when the project had none. */
     public function takeDown(string $projectId): bool
     {