Keep play start and game view scale in the project (#226)

Both settings lived in the browser's localStorage, so they did not
travel with the project and differed per machine. They now belong to
the project like the symbol scale. There is no migration, old projects
load with the defaults.

- New module src/core/model/playSettings.ts reads playFrom and
  gameScale on load. Unknown or missing values fall back to the start
  scene and 100%.
- src/editor/settings.ts is gone.
- Play starts in the project settings dialog is an undo step
  (SetPlayFromCommand). It is only recorded on a real change.
- The game view scale is saved with the project but is no undo step,
  so Ctrl+Z never undoes a zoom. A scale picked during play is written
  to the stored project and survives Stop.
- Save states of persistent Variables assets (saves.ts) stay in the
  browser on purpose. They are runtime data of the game, not settings.

Tests:
- 6 new unit tests in tests/editor/play-settings.test.ts: defaults,
  save and load, broken JSON, undo of Play starts, scale without undo
  step, scale picked during play survives Stop.
- e2e 03g checks the value in the project, nothing in localStorage and
  no new undo step. 35f also checks project.playFrom. No new checks,
  the sum stays at 1375.
- Counter checks: no write to the stored project in play, no read on
  load, and an undo step for the scale each fail exactly their test.
- ./check.sh: 158 files, 2332 tests, CHECK EXIT 0
- e2e 1-17,28,30,35,38-40,46: ALL PASSED, 752 checks

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

tests/core/holders.test.ts

@@ -152,6 +152,8 @@ describe('CommandStack', () => {
       plugins: [],
       collisionLayers: ['Default'],
       symbolScale: 1,
+      playFrom: 'start' as const,
+      gameScale: 1,
     };
     const stack = new CommandStack(project);
     let failing = false;