Various build shenenigans (#5)

This PR extends the build pipeline of the www service to support
locale-prefixed static builds (EN/RU) with build-time SSG output
injection, while also refactoring UI typography to a tokenized Tailwind
v4 @theme-driven approach.

Changes:

- Add build-time SSG plugin to @ermnvldmr/rsbuild-config and wire
multi-locale builds (/en/, /ru/) into services/www.
- Introduce new @ermnvldmr/i18n package (locale constants, path helpers,
keysets, pluralization) and apply it to www navigation/text.
- Refactor ui typography to a shared getTypographyClassNames utility +
Tailwind token definitions, with stl/cn updated for correct
Tailwind-merge behavior.

services/www/content/articles/2026/test-article.en.mdx

@@ -0,0 +1,15 @@
+---
+title: 'Hello World'
+description: 'My first article using the new content system.'
+createdDate: 2026-01-31
+tags: ['test', 'general']
+---
+
+## Section 1
+
+Some content here.
+
+```typescript
+const hello = 'world';
+console.log(hello);
+```