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.

packages/i18n/src/lib/keyset.test.tsx

@@ -0,0 +1,30 @@
+import React from 'react';
+import { describe, it, expect } from 'vitest';
+
+import { createKeyset } from './keyset.js';
+
+const { t, tRaw } = createKeyset({
+  greeting: { en: 'Hello', ru: 'Привет' },
+  'count {value}': { en: 'count {value}', ru: 'счёт {value}' },
+  items: {
+    en: { none: 'no items', one: 'one item', some: '{count} items', many: '{count} items' },
+    ru: { none: 'нет элементов', one: 'один элемент', some: '{count} элемента', many: '{count} элементов' },
+  },
+});
+
+describe('t', () => {
+  it('returns plain translation', () => expect(t('greeting')).toBe('Hello'));
+  it('interpolates {value}', () => expect(t('count {value}', { value: '42' })).toBe('count 42'));
+  it('pluralizes none', () => expect(t('items', { count: 0 })).toBe('no items'));
+  it('pluralizes one', () => expect(t('items', { count: 1 })).toBe('one item'));
+  it('pluralizes many with interpolation', () =>
+    expect(t('items', { count: 5 })).toBe('5 items'));
+});
+
+describe('tRaw', () => {
+  it('returns string node for plain key', () => expect(tRaw('greeting')).toBe('Hello'));
+  it('interpolates ReactNode', () => {
+    const node = tRaw('count {value}', { value: React.createElement('strong', null, '42') });
+    expect(Array.isArray(node)).toBe(true);
+  });
+});