Add a Russian translation and a theme switch to the page

[lesnik512]

Feedback from the first GitLab trial asked for Russian and a light theme.

• A language switch (English, Russian) and a theme switch (as in the system, light, dark) at the top of the page. The language starts from the browser's and both choices are kept in localStorage; the page works without it.
• Every text the page writes comes from one dictionary per language in index.html. Counts use Intl.PluralRules, so Russian gets its three forms ("1 задача", "2 задачи", "5 задач").
• The copied lists and the release post follow the page language, since the post goes to a team chat.
• The theme is applied in <head> before the first paint, so a stored choice does not flash. Dark colours are defined twice: under prefers-color-scheme: dark unless light is forced, and under [data-theme="dark"].

Not translated: warnings and errors from collect, and texts from GitLab and Jira (titles, statuses, summaries). Translating collector messages would need message codes in the report schema; GitLab and Jira errors cannot be translated by the page at all, and engineers search for them by their original wording.

Checked in a browser against a fixture report: both languages, all three themes, persistence across reloads, no console errors.