Header: language and mobile menus that work without JS, add README

[TheSeydiCharyyev]

Takes over #11 by @ahmedbatyrovv. His header design and his README stay; the fixes from the review on #11 are applied. Ahmed is a co-author of the commit.

Closes #5
Supersedes #11

What changes for visitors

• New header (Ahmed's design): a language menu with flags, and a menu button on narrow screens.
• Both menus work before the JavaScript loads. On a slow connection this takes several seconds.
• Keyboard: a closed menu is not in the Tab order. Escape closes an open menu and returns focus to its button.

Fixes from the #11 review

1. Flags load. They are in public/flags/, taken from lipis/flag-icons (MIT, license file included). The Turkmen flag is the official one, with the guls, the olive branches and the stars. It is a 60×45 PNG (2.5 KB), because the SVG is 38 KB and loads on every Turkmen page.
2. Both menus are <details> / <summary>. Without JS they open and close, and a closed menu cannot get focus.
3. HeaderMenus.tsx adds Escape, click outside, closing after a link in the mobile menu, and no page scroll while the mobile menu is open.
4. The header is a server component again; only the small menu script goes to the browser. JS per page: 173.6 KB (main: 173.2 KB, Update navbar design and created README.md file #11: 174.5 KB).
5. README: npm run check:size, the deploy section, the flag source.

One design change: the mobile menu had two close buttons, one in the panel and one on the dark area. Now there is one: the menu button turns into a close button on top of the dark area. It works without JS too.

Checked

• npm run build and npm run check:size pass.
• Chromium, desktop 1280 and phone 390: the flags load; every Tab stop is visible; Escape, a click outside, a tap on the dark area, a section link and the close button close the menus; a section link scrolls to the section; the page does not scroll while the mobile menu is open.
• JavaScript off: on desktop the section links are visible; on desktop and phone both menus open and close.
• Reduced motion: no transitions.
• <main> of /, /ru/, /projects/ and the 404 page are pixel-identical to production.