Let the game view show the picture at a chosen scale (#225)

The game view always stretched the camera picture to fit the pane.
Pixel art came out blurred at odd factors, and there was no way to
see the game at its real size. The view now has a header with a
scale drop-down, and 100 % is the default.

- The header of the new GamePane offers Auto, Fit, 25 %, 50 %,
  100 %, 200 % and 400 %. Auto and Fit show the factor they land
  on, e.g. "Auto · 100%", and follow the pane when it resizes.
- 100 % means one screen pixel per pixel of the camera resolution
  (width x height). The camera zoom scales the world on top of
  that, so a zoomed camera still fills the same frame.
- Auto takes the largest fixed step that fits the whole picture
  and falls back to 25 % when nothing fits. Fit keeps the old
  behaviour.
- A picture larger than the pane is cropped around the camera
  centre. Fixed steps start on a whole pixel so pixel art stays
  sharp at 100 %.
- The choice is kept in the browser like "Play starts". Nothing
  is written into the project.

Found on the way: the first draft applied the step to the world
instead of the camera resolution. The camera zoom then had no
effect at 100 %, which the existing check 12c caught (zoom 1.37
measured as scale 1).

The CI smokes were not run locally, because that would reseed
projects the user works in. All three smoke cameras are 640x360
and fit the 670x383 harness view at 100 %.

Tests:
- ./check.sh: 156 files, 2298 tests, CHECK EXIT 0 (before 2290).
- 8 unit tests in tests/viewport/view.test.ts for whole-pixel
  steps, cropping, Auto and Fit with fallback, and zoomed cameras.
- 7 e2e checks in 03g. e2e 1-17,28,30,38-40: ALL PASSED, 727
  checks.
- Counter-checks: no cropping, no rounding, no factor reported to
  the header, and no zoom share each fail their own tests.
- Check total: 1367 (CI at c1a702b) + 7 = 1374, computed only.

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, 1367 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1374 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