Show only the game view in fullscreen, in the editor and the link (#267)

A game is easier to judge and to play without the editor around it.
The Game tab and the game link now both have a button that puts the
game view itself into fullscreen, not just the browser window.

- The Game tab header has a fullscreen button on the right. It hides
  toolbar, panels, tab headers and console and keeps the zoom from the
  picker. A running game keeps running.
- A corner button at the top right, or Esc, leaves fullscreen and puts
  the view back into the tab where it was.
- The game link shows the same corner button. Over a running game it
  hides after 2 s without mouse movement and returns on the next move.
- A click on the corner button never reaches the game.
- Stop ends fullscreen. Stop switches back to the Scene tab, and any
  switch away from the Game tab ends fullscreen, so no empty screen is
  left behind.
- src/viewport/fullscreen.ts wraps the Fullscreen API and holds the
  rule for when the button shows. FullscreenCorner.ts is the button
  used by both pages. Two SVG icons are new.
- Editor.showGameFullscreen moves the GameViewport into the root while
  fullscreen is on.

Tests:
- ./check.sh: 195 files, 2815 tests, exit 0 (3 new unit tests).
- e2e 60: 2 new checks for the corner button on the game link.
- e2e 61 (new): 4 checks for fullscreen in the editor.
- Full e2e run (local): ALL PASSED, 1580 checks.
- Counter-checks fail as expected: rule without "no game running",
  no stopPropagation (60h), no end on tab switch (61c), no reparenting
  of the view (61a).

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

src/player/GameScreen.ts

@@ -2,6 +2,7 @@ import { Text, type TextStyle } from 'pixi.js';
 import type { World } from '../runtime/World';
 import { Theme, textStyle, ui } from '../ui';
 import { cameraOfWorld } from '../viewport/activeCamera';
+import { FullscreenCorner } from '../viewport/FullscreenCorner';
 import { routePointer } from '../viewport/gamePointer';
 import { collectWorldItems } from '../viewport/items';
 import { cameraView, type View } from '../viewport/view';
@@ -27,6 +28,7 @@ export class GameScreen extends WorldViewport {
   /** Asked when the caption is clicked while it offers to start. */
   onStart: (() => void) | null = null;
   private startable = false;
+  readonly fullscreenButton = new FullscreenCorner(this);
 
   constructor(source: ViewSource) {
     super(source);
@@ -35,6 +37,7 @@ export class GameScreen extends WorldViewport {
       text.anchor.set(0.5);
       this.addChild(text);
     }
+    this.addChild(this.fullscreenButton);
     routePointer(
       this,
       () => this.running,
@@ -59,6 +62,7 @@ export class GameScreen extends WorldViewport {
   /** null = not running. */
   setWorld(world: World | null): void {
     this.running = world;
+    this.fullscreenButton.sync(true, world !== null);
     this.redraw();
   }
 
@@ -101,5 +105,6 @@ export class GameScreen extends WorldViewport {
     this.caption.position.set(this.w / 2, this.h / 2);
     this.hint.visible = this.running !== null && view === null;
     this.hint.position.set(this.w / 2, this.h / 2);
+    this.fullscreenButton.place(this.w);
   }
 }

src/player/playerHook.ts

@@ -13,5 +13,14 @@ export function installPlayerHook(screen: GameScreen, player: () => Player | nul
     screen: () => screen.info(),
     log: () => player()?.log() ?? [],
     pixelAt: (x: number, y: number) => stagePixel(ui().app, x, y),
+    /** Whether the page is in fullscreen, and the corner button while it shows (#267). */
+    fullscreen: () => {
+      const button = screen.fullscreenButton;
+      const origin = button.getGlobalPosition();
+      return {
+        on: document.fullscreenElement !== null,
+        button: button.visible ? { x: origin.x + button.w / 2, y: origin.y + button.h / 2 } : null,
+      };
+    },
   };
 }