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>

src/editor/Editor.ts

@@ -4,6 +4,7 @@ import { findAsset, findObjectHolder, ROUTING_ID } from '../core/model/Project';
 import { assetTitle } from '../core/music/embedded';
 import { mixOf, videoBusesOf } from '../core/music/mixer';
 import type { Role } from '../core/store/AuthStore';
+import type { DemoStore } from '../core/store/DemoStore';
 import type { ProjectStore } from '../core/store/ProjectStore';
 import { pluginsOf } from '../core/store/projectPlugins';
 import type { FontStore } from '../core/store/FontStore';
@@ -158,7 +159,11 @@ export interface EditorAccount {
   user: string;
   /** What the editor offers: an administrator gets the user management (#180, #179). */
   role: Role;
+  /** Somebody trying a demo copy without an account (#428): nothing they do is kept. */
+  guest?: boolean;
   users: UsersStore;
+  /** An administrator's: making the demo copy of the open project (#428). */
+  demos?: DemoStore;
   logOut: () => void;
   /** Rejects with `WrongCredentialsError` when the current password does not match (#182). */
   changePassword: (current: string, next: string) => Promise<void>;
@@ -365,6 +370,7 @@ export class Editor implements PlayHost {
   private readonly cleanups: (() => void)[] = [];
   private readonly autosave: { flush: () => Promise<void>; stop: () => void };
   private disposed = false;
+  private readonly guest: boolean;
   // Each starts where its divider was left in this browser (`panelSizes.ts`), else at the number here.
   private leftWidth = panelSize('left', 250, MIN_LEFT_WIDTH, MAX_LEFT_WIDTH);
   private rightWidth = panelSize('right', 350, MIN_RIGHT_WIDTH, MAX_RIGHT_WIDTH);
@@ -448,11 +454,13 @@ export class Editor implements PlayHost {
       () => {
         if (this.play.isPlaying) this.play.stop();
       },
-      { snapshots, releases, flush: () => this.autosave.flush() },
+      { snapshots, releases, demos: account.demos, flush: () => this.autosave.flush() },
+      { guest: account.guest === true, role: account.role },
       // Every project brings its own plugins (#119); until `startPlugins` has run there is no
       // manager, and a project with plugins simply has none of them.
       async (raw) => void (await this.plugins?.syncTo(pluginsOf(raw))),
     );
+    this.guest = account.guest === true;
 
     this.graphEditor.canvas.traceProvider = (assetId) => this.play.traceFor(assetId);
     this.graphEditor.canvas.timeProvider = () => this.play.time;
@@ -476,16 +484,20 @@ export class Editor implements PlayHost {
         : null;
     this.managementPane = new ManagementPane(this.usersPane, this.pluginsPane);
 
-    this.toolbar = new Toolbar(state, {
-      openMainMenu: (anchor) =>
-        this.projects.openMainMenu(anchor, this.managementEntries(), [
-          { label: 'Master routing', onSelect: () => this.openDocument('routing') },
-        ]),
-      togglePlay: () => (state.mode === 'edit' ? this.play.play() : this.play.stop()),
-      togglePause: () => this.play.togglePause(),
-      step: () => this.play.step(),
-      openAccountMenu: (anchor) => this.openAccountMenu(anchor, account),
-    });
+    this.toolbar = new Toolbar(
+      state,
+      {
+        openMainMenu: (anchor) =>
+          this.projects.openMainMenu(anchor, this.managementEntries(), [
+            { label: 'Master routing', onSelect: () => this.openDocument('routing') },
+          ]),
+        togglePlay: () => (state.mode === 'edit' ? this.play.play() : this.play.stop()),
+        togglePause: () => this.play.togglePause(),
+        step: () => this.play.step(),
+        openAccountMenu: (anchor) => this.openAccountMenu(anchor, account),
+      },
+      account.guest === true,
+    );
     root.add(this.toolbar);
 
     this.workspace = new HStack();
@@ -614,7 +626,10 @@ export class Editor implements PlayHost {
     // A press in the editor takes the keys back from a window (#371).
     this.workspace.on('pointerdowncapture', () => (this.pressedWindow = null));
 
-    this.autosave = attachAutosave(state, store);
+    // A guest's project lives in the tab alone (#428): there is nowhere to save it to.
+    this.autosave = this.guest
+      ? { flush: () => Promise.resolve(), stop: () => {} }
+      : attachAutosave(state, store);
     this.cleanups.push(
       installShortcuts(state, {
         play: () => this.play.play(),
@@ -863,6 +878,14 @@ export class Editor implements PlayHost {
    * picked: it says who is signed in without costing the crowded toolbar another label.
    */
   private openAccountMenu(anchor: Widget, account: EditorAccount): void {
+    if (this.guest) {
+      new Menu([
+        { label: 'Guest – nothing is saved', disabled: true, onSelect: () => {} },
+        { separator: true },
+        { label: 'Sign in…', onSelect: () => account.logOut() },
+      ]).openBelow(anchor);
+      return;
+    }
     new Menu([
       { label: `Signed in as ${account.user}`, disabled: true, onSelect: () => {} },
       { separator: true },
@@ -1348,8 +1371,13 @@ export class Editor implements PlayHost {
     };
   }
 
-  boot(): Promise<void> {
-    return this.projects.boot();
+  async boot(): Promise<void> {
+    await this.projects.boot();
+    if (this.guest) {
+      this.state.setNotice(
+        'You are trying a demo as a guest: change anything, play it, record it – nothing is saved. Sign in from the menu at the far right to keep your work.',
+      );
+    }
   }
 
   /**