Fix crashes from the October 9 error reports

[Mistium]

Fixes from the October 9 error report export (60 open reports). Every report came from a build that already included #129, so these are new problems rather than repeats.

What changed

• Safari chunk failures (5 reports, iPad). Safari reports a blocked dynamic import as "Cross-origin script load denied by Cross-Origin Resource Sharing policy." That message was not recognised as a chunk load error, so community pages crashed without a retry. All five reports came from iPads within 13 minutes, which looks like a school network filter. These failures now retry, then reload once. A page that still fails shows a "could not load" screen. Its Try again now really starts a new import (before, React.lazy cached the failure), and the failure is not reported as a crash.
• Edge removeChild crashes (8 reports, all Edge 154). index.ejs and embed.ejs had <meta name="google" value="notranslate">, which browsers ignore. It now uses content=, and the <html> element has translate="no", so the editor and embed are no longer machine-translated under React. The community site, which has a separate template, can still be translated.
• DOMElementRenderer only removes the stage canvas if it still contains it (2 of those 8 reports).
• Sprite list: SortableHOC no longer crashes with "Cannot destructure property 'top' of 'this.containerBox'" when it mounts in the middle of a drag.
• No WebGL: restore points now save with a blank thumbnail instead of failing every time, so the leftover timer no longer throws reading 'draw'. The gamepad addon also stops early instead of throwing reading 'canvas'.
• Missing extensions (4 reports, embed): a project that needs an extension MistWarp does not have, such as p7videosharing from another Scratch mod, now gets a load error that names the extension instead of the crash screen. Embeds no longer offer "Start a new project", which would open the editor inside the project page. The load error screen now also fills the embed.

Engine fixes from the same export are in MistWarp/scratch-blocks and MistWarp/scratch-paint PRs on the oct10-error-fixes branch.

Treated as noise, not changed: third-party extension errors (Shaders.js, roturVoice Advanced.js, Canvas.js, data: URL extensions, the Lily ClonesPlus.js load, and lab/text.js reading the null that makeClone returns at the clone limit, which TurboWarp does too), maze recursion, appVersion: dev, and the stale chunk on the ux-bug-pass-oct preview.

Screenshots

Editor opening a project that needs a missing extension:

The same screen in the dark theme:

The same project in an embed, with no "Start a new project" link:

A community page whose code chunk was blocked:

Testing

• New unit tests for the sortable mount, DOMElementRenderer unmount, the missing-extension screen, the embed screen and the Safari message. The sortable test fails without the fix with the reported error.
• Full CI steps run locally: lint, both i18n checks, check:community-css, deps:check, test:unit:ci (2453 passed), build:deploy and validate-deploy.

Preview: https://oct10-error-fixes.scratch-gui.pages.dev/editor (for the missing-extension screen, open the editor with ?project_url= pointing at a project.json whose extensions lists an unknown ID).

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