Add zoom steps and a larger start caption to the game link (#266)

The released game only ever zoomed in whole steps up to 400%, so a
small camera on a large screen stayed far smaller than it could be.
The "Click to play" caption was drawn in the 11 px panel title size
and was easy to miss.

- GAME_SCALE_STEPS gains 300%, 500% and 600%. The game tab picker,
  Auto in the editor and the game link all read the same list.
- The start caption takes its size from the new Theme.captionSize
  (16) instead of a constant in GameScreen.
- The player page keeps the default arrow cursor at all times. It no
  longer switches to the hand over the caption.

Tests:
- view.test: Auto for 320x180 in 2000x2000 is now 6, with cases
  for 3 and 5. play-settings.test takes 2.5 and 8 as invalid steps.
- e2e 60 checks the caption against the panel title size and the
  arrow cursor before and after the start. Module 03 follows the new
  step list.
- ./check.sh 2808 tests, CHECK EXIT 0 (before the last Theme tweak,
  which changes no test). Full e2e run ALL PASSED, 1574 checks.

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

README.md

@@ -43,7 +43,7 @@ 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, 1572 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1574 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