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.ts

@@ -0,0 +1,132 @@
+import { LOCALE } from './locale.js';
+import { pluralize, type PluralForms } from './pluralize.js';
+
+import type { ReactNode } from 'react';
+
+/** A translation entry with a plain string per locale. */
+interface PlainTranslation { en: string; ru: string }
+
+/** A translation entry with plural forms per locale. */
+interface PluralTranslation { en: PluralForms; ru: PluralForms }
+
+/** Union of all supported translation shapes. */
+type Translation = PlainTranslation | PluralTranslation;
+
+/** A map of translation keys to their translation entries. */
+type Keyset = Record<string, Translation>;
+
+/**
+ * Returns `true` when the translation entry uses plural forms.
+ *
+ * @param t - The translation entry to inspect.
+ * @returns Type predicate narrowing `t` to {@link PluralTranslation}.
+ * @example
+ * isPluralTranslation({ en: 'hello', ru: 'привет' }) // → false
+ */
+function isPluralTranslation(t: Translation): t is PluralTranslation {
+  return typeof t.en === 'object';
+}
+
+/**
+ * Replaces `{key}` placeholders in `template` with values from `params`.
+ *
+ * @param template - String containing `{key}` placeholders.
+ * @param params - Map of placeholder names to string/number values.
+ * @returns The interpolated string.
+ * @example
+ * interpolate('Hello {name}!', { name: 'World' }) // → 'Hello World!'
+ */
+function interpolate(template: string, params: Record<string, string | number>): string {
+  return template.replace(/\{(\w+)\}/g, (_, key: string) =>
+    key in params ? String(params[key]) : `{${key}}`
+  );
+}
+
+/**
+ * Splits `template` on `{key}` placeholders and substitutes ReactNode values.
+ *
+ * @param template - String containing `{key}` placeholders.
+ * @param params - Map of placeholder names to ReactNode values.
+ * @returns A single string when all parts are strings, otherwise a ReactNode array.
+ * @example
+ * interpolateRaw('Click {link}', { link: <a href="/">here</a> })
+ */
+function interpolateRaw(template: string, params: Record<string, ReactNode>): ReactNode {
+  const parts = template.split(/(\{[^}]+\})/g);
+  const result = parts.map((part) => {
+    const match = /^\{(\w+)\}$/.exec(part);
+    if (match) {
+      const key = match[1];
+      return key in params ? params[key] : part;
+    }
+    return part;
+  });
+  if (result.every((p): p is string => typeof p === 'string')) {
+    return result.join('');
+  }
+  return result;
+}
+
+/**
+ * Creates a typed `t` and `tRaw` pair bound to `keyset`.
+ *
+ * @param keyset - An object mapping translation keys to their locale entries.
+ * @returns `{ t, tRaw }` translation helpers.
+ * @example
+ * const { t } = createKeyset({ greeting: { en: 'Hello', ru: 'Привет' } });
+ * t('greeting') // → 'Hello'  (in EN build)
+ */
+export function createKeyset<const K extends Keyset>(keyset: K) {
+  /** String union of all keys defined in `keyset`. */
+  type Keys = keyof K & string;
+
+  /**
+   * Returns the translated string for `key`, with optional interpolation and pluralisation.
+   *
+   * @param key - A key defined in the keyset.
+   * @param params - Optional interpolation/pluralisation params.
+   * @returns The translated string for the current build locale.
+   * @example
+   * t('greeting')                        // → 'Hello'
+   * t('items {count}', { count: 3 })     // → '3 items'
+   */
+  function t(key: Keys, params?: Record<string, string | number>): string {
+    const entry = keyset[key];
+
+    if (isPluralTranslation(entry)) {
+      const forms = entry[LOCALE];
+      const count = typeof params?.count === 'number' ? params.count : 0;
+      const template = pluralize(count, forms);
+      return params ? interpolate(template, params) : template;
+    }
+
+    const template = entry[LOCALE];
+    return params ? interpolate(template, params) : template;
+  }
+
+  /**
+   * Returns the translated ReactNode for `key`, allowing ReactNode interpolation values.
+   *
+   * @param key - A key defined in the keyset.
+   * @param params - Optional interpolation params; values may be any ReactNode.
+   * @returns The translated string or ReactNode array for the current build locale.
+   * @example
+   * tRaw('click {link}', { link: <a href="/">here</a> })
+   */
+  function tRaw(key: Keys, params?: Record<string, ReactNode>): ReactNode {
+    const entry = keyset[key];
+
+    if (isPluralTranslation(entry)) {
+      const forms = entry[LOCALE];
+      const countParam = params?.count;
+      const count = typeof countParam === 'number' ? countParam : 0;
+      const template = pluralize(count, forms);
+      return params ? interpolateRaw(template, params) : template;
+    }
+
+    const template = entry[LOCALE];
+    return params ? interpolateRaw(template, params) : template;
+  }
+
+  return { t, tRaw };
+}