Let ?w=&h= fix the editor canvas to a set size

A recording of the editor is as large as its canvas, and so far the
canvas always followed the window. With ?w=1920&h=1080 the editor is
laid out at exactly that size in CSS pixels, whatever the window.

- src/ui/pageSize.ts reads w and h (both needed, 200 to 8192 each) and
  works out how the canvas fits the window.
- A smaller window shows the canvas shrunk to fit, centred, with the
  same aspect. A larger one shows it 1:1, centred on a dark page.
- Clicks still land right, since Pixi maps them over the canvas's
  on-screen box. Switching projects keeps w and h in the URL.
- The pointer drawn into an editor recording now scales against the
  layout size, so a shrunk canvas still records a full-size pointer.

Tests:
- Unit page-size: 5 new, green.
- e2e 62d: ?w=1600&h=900 in a 960x720 window gives a 1600x900 canvas
  shown at 960x540, y 90, and a click on the menu button opens it.
  Red without the shrinking.
- Gate on melektaus-atx: 304 files, 4691 tests green.

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

README.md

@@ -47,7 +47,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2039 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2040 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs