Guard against misconfiguration: publishing a translation overwrites the source page when canvas_page is not translatable

Summary

Canvas ships canvas_page base fields as non-translatable, and nothing in Canvas Translate checks that a site has actually enabled translation for pages. On a misconfigured site (setup never done, or silently reverted by a config import), the editor and publish flow work normally, but every write to the "translation" lands on the field storage shared between all languages: publishing silently overwrites the original language.

The risk is that the misconfiguration is invisible until the damage is done. Nothing fails, nothing warns; the translator only notices when the source page comes back translated.

Steps to reproduce

1. Translate any page in the Canvas Translate editor and publish.
2. The default-language page now shows the translated text.

Proposed resolution

Detect the misconfiguration and refuse before anything is written:

1. When page translation cannot work at all (bundle not translation-enabled, or no writable field translatable), every endpoint refuses with 409, code: translation_not_enabled, and the SPA hides the whole workspace behind setup instructions. The status report flags it via hook_runtime_requirements() (error on multilingual sites).
2. Publish additionally never writes a non-translatable field (guard at the write site), so no future path can reintroduce the clobber.