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/FileUsage.php

@@ -4,8 +4,9 @@ declare(strict_types=1);
 /**
  * Which projects still use a stored image, font or sound (#344) – asked before one is deleted.
  *
- * A project counts when the file is named in the project itself, in one of its snapshots or in its
- * release: restoring a snapshot or playing a released game must not find a hole. Any string value
+ * A project counts when the file is named in the project itself, in one of its snapshots, in its
+ * release or in its demo copy: restoring a snapshot, playing a released game or trying a demo must
+ * not find a hole. Any string value
  * equal to the stored name counts, wherever in the document it is; that may refuse a file nothing
  * really draws, but never lets one go that something does.
  */
@@ -31,7 +32,8 @@ final class FileUsage
                 $used[basename(dirname($path))] = true;
             }
         }
-        foreach (glob("{$this->data}/releases/*.json") ?: [] as $path) {
+        // A demo copy (#428) is a release a guest opens in the editor, and stored like one.
+        foreach ([...glob("{$this->data}/releases/*.json") ?: [], ...glob("{$this->data}/demos/*.json") ?: []] as $path) {
             $release = $this->decoded($path, $file);
             if ($release !== null && is_string($release['projectId'] ?? null) && self::uses($release['project'] ?? null, $file)) {
                 $used[$release['projectId']] = true;
@@ -74,7 +76,7 @@ final class FileUsage
             $named[] = $others === 1 ? 'a project of another account' : "{$others} projects of other accounts";
         }
         $list = count($named) > 1 ? implode(', ', array_slice($named, 0, -1)) . ' and ' . end($named) : $named[0];
-        return "Still used by {$list}, counting snapshots and releases";
+        return "Still used by {$list}, counting snapshots, releases and demo copies";
     }
 
     /** Whether a value holds `$file` as a string anywhere inside it. */