Keep where each divider was dragged to across a reload

A panel dragged wider was back at its first size after every reload.
The sizes are now kept in the browser, next to the folded groups.

- editor/panelSizes.ts holds one entry in localStorage,
  game-maker.panel-sizes: a name per divider and its width, height
  or share.
- A panel reads its size once when it is built, held to the bounds
  its divider keeps today. Every drag writes the new size.
- Kept are the columns left and right of the centre, the console,
  the split of the left column, the tilemap panel, the preview of
  the Shader, Particles and Pixel art tabs, each under its own name,
  and the split of the user management.
- The sizes belong to the browser, not to the project. They hold for
  every project opened there.
- A browser that refuses the storage, or holds rubbish under the
  entry, starts with the sizes the editor comes with.

Tests:
- Gate on melektaus-atx: 219 files, 3400 tests, CHECK EXIT 0.
- Seven new unit tests in tests/editor/panel-sizes.test.ts. All
  five counter-probes bite.
- Smokes on melektaus-atx: 24, 19 and 21, all pass.
- One new e2e check in module 48: the left column dragged 40 px
  wider is as wide after a reload, and dragged back after.
- Full suite on melektaus-atx: 1692 checks, all pass. It ran before
  the fix to the animation players of the commit before last.
- Browser counter-probe: without the write to the browser exactly
  that check fails.
- Only the left column is measured across a reload. The other
  dividers go the same way and have no check of their own.

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

README.md

@@ -45,7 +45,7 @@ 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, 1691 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1692 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