Migrate site to Next.js with GitHub Actions deploy

[TheSeydiCharyyev]

Moves the organization site from hand-written HTML to Next.js (static export) and deploys it with GitHub Actions.

What changes for visitors

• Same design and texts. Home page is pixel-identical to the current site at 1280px.
• Languages are now addresses: / is Turkmen, /ru/ and /en/ are Russian and English. Each page has its own <html lang>, canonical, hreflang and Open Graph tags, so shared links preview in the right language.
• /projects/ works in all three languages. Star counts are live, as before.
• One 404 page in three languages. sitemap.xml is generated at build time with hreflang for every page.
• /hytay-dili-1/privacy-policy.html stays at the same address (it is in the App Store listing).

How it is built and deployed

• .github/workflows/deploy.yml: every PR to main builds the site; every push to main builds and deploys to Pages. Only main can deploy (github-pages environment policy).
• The build fails if a page the stores depend on is missing from the output, including the privacy policy.

Known costs

• JavaScript per page grows from about 3 KB to about 174 KB gzip (React runtime). Text is in the HTML and shows before the JS loads.
• global-not-found is still an experimental Next.js flag. It is needed because the site has two root layouts (Turkmen without prefix, other languages with prefix).

Switch order

1. This PR builds green.
2. Settings → Pages → Source: GitHub Actions. The current site stays live until the first new deploy.
3. Merge. The workflow deploys in 1–2 minutes.

Rollback: revert the merge commit and set Pages Source back to "Deploy from a branch".