Port the UI restyle to the Russian docs

[glebfox]

Brings the docs UI restyle and dark theme from feature/ui-restyle (#203) to the Russian docs.

release_3_1_ru shares no history with the English branches, so the restyle is applied as a patch (release_3_1..feature/ui-restyle) instead of a merge.

Russian-specific changes on top of the restyle:

• The header keeps Russian labels, the docs.jmix.ru, ai-assistant.jmix.ru and www.jmix.ru links, the event banner and the "Documentation in English" link, now an inline icon like the other header icons.
• The Russian strings from the removed overrides.css (pagination labels, the add-on admonition caption) move to part 2 of site.css.
• New UI strings are translated: the color theme menu, the skip link, the copy button name, the navigation toggles and the footer.
• The feedback form stays out, as before; its checks are removed from tools/ui-audit.mjs and its mentions from AGENTS.md.
• lunr-languages.js and the Russian "Edit this page" link are kept.

Checked: node tools/check-css.mjs and its tests, a local build, and node tools/ui-audit.mjs (all checks pass in both themes).

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