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/dev.ts

@@ -0,0 +1,60 @@
+import { DEFAULT_LOCALE, isLocale, type Locale } from './lib/locale.js';
+
+import type { RsbuildConfig } from '@rsbuild/core';
+
+/**
+ * Returns a partial Rsbuild config that enables locale-aware builds.
+ *
+ * Designed to be merged with defineServiceConfig() in each service's rsbuild.config.ts:
+ *
+ * @example
+ * ```ts
+ * import { localeRsbuildConfig } from '@ermnvldmr/i18n/dev';
+ * import { defineServiceConfig, mergeConfig } from '@ermnvldmr/rsbuild-config/dev';
+ *
+ * export default defineServiceConfig(mergeConfig(localeRsbuildConfig(), { ... }));
+ * ```
+ */
+export function localeRsbuildConfig(): RsbuildConfig {
+  const rawLocale = process.env.LOCALE ?? '';
+  const locale: Locale = isLocale(rawLocale) ? rawLocale : DEFAULT_LOCALE;
+
+  return {
+    source: {
+      define: {
+        'process.env.LOCALE': JSON.stringify(locale),
+      },
+    },
+    server: {
+      // Dev server serves pages under the locale base so localePath() links
+      // resolve correctly in dev without any NODE_ENV branching.
+      base: `/${locale}`,
+    },
+    output: {
+      distPath: {
+        root: `dist/${locale}`,
+      },
+      assetPrefix: `/${locale}/`,
+    },
+    tools: {
+      rspack: (config, { rspack }) => {
+        // Resolve .en.mdx imports to the locale-specific file when LOCALE != 'en'.
+        // The canonical import is always `.en.mdx`; other locales are resolved here.
+        if (locale !== DEFAULT_LOCALE) {
+          config.plugins = [
+            ...config.plugins,
+            new rspack.NormalModuleReplacementPlugin(
+              /\.en\.mdx$/,
+              (resource: { request: string }) => {
+                resource.request = resource.request.replace(/\.en\.mdx$/, `.${locale}.mdx`);
+              }
+            ),
+          ];
+        }
+        return config;
+      },
+    },
+  };
+}
+
+export { LOCALES, DEFAULT_LOCALE, type Locale } from './lib/locale.js';