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/FilePicker.ts

@@ -26,6 +26,7 @@ import {
   crispText,
 } from '../ui';
 import { dialogButtons } from './FormDialog';
+import { allows } from './session';
 
 /** The dialog keeps this size whatever the list turns out to be, so it does not jump on opening. */
 export const PICKER_WIDTH = 560;
@@ -55,10 +56,15 @@ function ownerText(owner: string | null): string {
 /** The line under the list for a picked file: its upload name and size, whose it is, and who wears it. */
 function describeStored(item: PickerItem): string {
   const size = item.name ? `${item.name} – ${formatBytes(item.bytes)}` : formatBytes(item.bytes);
-  const owned = `${size} – ${ownerText(item.owner)}`;
+  const owned = `${size} – ${ownerText(item.owner)}${demoText(item)}`;
   return item.assetName ? `${owned} – already in the project as "${item.assetName}"` : owned;
 }
 
+/** Said after what a file is when an administrator offers it to everybody (#428). */
+export function demoText(item: StoredFile): string {
+  return item.demo ? ' – demo material' : '';
+}
+
 export interface PickerRow {
   /** A Container holding the rest, not a Graphics with children: Pixi v8 only lets containers have them. */
   box: Container;
@@ -116,6 +122,10 @@ export abstract class PickerList<Item extends PickerItem, Row extends PickerRow>
     return this.items.filter((item) => item.assetName !== null).map((item) => item.file);
   }
 
+  get demoFiles(): string[] {
+    return this.items.filter((item) => item.demo === true).map((item) => item.file);
+  }
+
   /** The captions as drawn, read off the rows themselves rather than recomputed (test hook). */
   get captions(): string[] {
     return this.rows.map((row) => row.caption.text);
@@ -203,6 +213,16 @@ export abstract class RowList<Item extends PickerItem, Row extends PickerRow> ex
 
 /** What the Delete button says while it waits for the second press that really deletes. */
 const ARMED_LABEL = 'Delete for good';
+const OFFER_LABEL = 'Offer as demo';
+const WITHDRAW_LABEL = 'Stop offering';
+
+interface ButtonInfo {
+  enabled: boolean;
+  label: string;
+  shown: string;
+  x: number;
+  y: number;
+}
 
 export interface PickerWords {
   title: string;
@@ -243,6 +263,8 @@ export abstract class FilePicker<Item extends PickerItem> extends Popup {
   private readonly detail: Label;
   private readonly use: Button;
   private readonly remove: Button;
+  /** An administrator's switch for offering the picked file as demo material (#428). */
+  private readonly offer: Button | null;
   private readonly buttons: HStack;
   /** The file the Delete button was pressed for once; the second press deletes it. */
   private armed: string | null = null;
@@ -254,6 +276,8 @@ export abstract class FilePicker<Item extends PickerItem> extends Popup {
     private readonly deleteFile: (file: string) => Promise<void>,
     /** Buttons of a kind of picker of its own, after Delete; `picked` says what is picked. */
     protected readonly extras: readonly Button[] = [],
+    /** There for an administrator only, see `FileStore.setDemo`. */
+    private readonly setDemo?: (file: string, demo: boolean) => Promise<void>,
   ) {
     super({ modal: true, padding: Theme.padding * 2, gap: Theme.gap });
     this.content.add(new Label(words.title, { style: 'bodyBold' }));
@@ -280,36 +304,52 @@ export abstract class FilePicker<Item extends PickerItem> extends Popup {
     this.remove = new Button('Delete', { variant: 'danger' });
     this.remove.onClick = () => void this.pressDelete();
     this.remove.setEnabled(false);
-    this.buttons = dialogButtons(this.closeThen(), this.use, [uploadButton, this.remove, ...extras]);
+    this.offer = setDemo ? new Button(OFFER_LABEL) : null;
+    if (this.offer) {
+      this.offer.onClick = () => void this.toggleDemo();
+      this.offer.setEnabled(false);
+    }
+    // A guest (#428) uploads and deletes nothing: the buttons are not there rather than refused.
+    const writes = allows('upload') ? [uploadButton, this.remove] : [];
+    this.buttons = dialogButtons(this.closeThen(), this.use, [
+      ...writes,
+      ...extras,
+      ...(this.offer ? [this.offer] : []),
+    ]);
     this.content.add(this.buttons);
   }
 
   /**
    * What the dialog offers, which of it the project already wears, what is picked, the line under
-   * the list and the Delete button with its centre (test hook).
+   * the list, the Delete button with its centre and, for an administrator, the demo switch; a
+   * button that is not in the dialog is null (test hook).
    */
   debugInfo(): {
     files: string[];
     used: string[];
+    demo: string[];
     captions: string[];
     selected: string | null;
     detail: string;
-    deleteButton: { enabled: boolean; label: string; shown: string; x: number; y: number };
+    deleteButton: ButtonInfo | null;
+    uploadShown: boolean;
+    offerButton: ButtonInfo | null;
   } {
-    const at = this.remove.getGlobalPosition();
+    const info = (button: Button): ButtonInfo => {
+      const at = button.getGlobalPosition();
+      const centre = { x: at.x + button.w / 2, y: at.y + button.h / 2 };
+      return { enabled: button.enabled, label: button.text, shown: button.shownText, ...centre };
+    };
     return {
       files: this.list.fileList,
       used: this.list.usedFiles,
+      demo: this.list.demoFiles,
       captions: this.list.captions,
       selected: this.list.selected?.file ?? null,
       detail: this.detail.text,
-      deleteButton: {
-        enabled: this.remove.enabled,
-        label: this.remove.text,
-        shown: this.remove.shownText,
-        x: at.x + this.remove.w / 2,
-        y: at.y + this.remove.h / 2,
-      },
+      deleteButton: this.remove.parent ? info(this.remove) : null,
+      uploadShown: this.remove.parent !== null,
+      offerButton: this.offer ? info(this.offer) : null,
     };
   }
 
@@ -352,6 +392,11 @@ export abstract class FilePicker<Item extends PickerItem> extends Popup {
     this.setDeleteLabel('Delete');
     this.use.setEnabled(item !== null && this.canUse(item));
     this.remove.setEnabled(item?.deletable ?? false);
+    if (this.offer) {
+      this.offer.setEnabled(item !== null);
+      this.offer.setText(item?.demo ? WITHDRAW_LABEL : OFFER_LABEL);
+      this.buttons.layout();
+    }
     this.picked(item);
     this.detail.setText(item ? this.describe(item) : (this.words.idle ?? ''));
     this.refreshLayout();
@@ -401,6 +446,27 @@ export abstract class FilePicker<Item extends PickerItem> extends Popup {
     this.refreshLayout();
   }
 
+  /** Offers the picked file to everybody as demo material, or takes that back (#428). */
+  private async toggleDemo(): Promise<void> {
+    const item = this.list.selected;
+    if (!item || !this.setDemo) return;
+    const offered = !item.demo;
+    try {
+      await this.setDemo(item.file, offered);
+    } catch (error) {
+      this.say(`Not changed: ${error instanceof Error ? error.message : String(error)}`);
+      return;
+    }
+    if (this.list.destroyed) return;
+    await this.load();
+    this.list.select(item.file);
+    this.say(
+      offered
+        ? `Every account and every guest can use ${item.name || `this ${this.words.noun}`} now.`
+        : `${item.name || `This ${this.words.noun}`} is no longer offered as demo material.`,
+    );
+  }
+
   private confirm(): void {
     const item = this.list.selected;
     if (!item || !this.canUse(item)) return;