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/stl/src/lib/functions/cn/cn.ts

@@ -1,5 +1,20 @@
 import { clsx, type ClassValue } from 'clsx';
-import { twMerge } from 'tailwind-merge';
+import { extendTailwindMerge } from 'tailwind-merge';
+
+const RB_TYPES = ['display', 'headline', 'title', 'body', 'label'] as const;
+const RB_SIZES = ['l', 'm', 's'] as const;
+
+const rbFontSizeClasses = RB_TYPES.flatMap((type) =>
+  RB_SIZES.map((size) => `${type}-${size}`)
+);
+
+const twMerge = extendTailwindMerge({
+  extend: {
+    classGroups: {
+      'font-size': [{ 'text-rb': rbFontSizeClasses }],
+    },
+  },
+});
 
 /**
  * Combines and merges CSS class names using clsx and tailwind-merge.
@@ -7,6 +22,10 @@ import { twMerge } from 'tailwind-merge';
  * This utility function combines the power of clsx for conditional class handling
  * with tailwind-merge for proper Tailwind CSS class deduplication and conflict resolution.
  *
+ * Tailwind-merge is configured to recognise `text-rb-{type}-{size}` classes
+ * (e.g. `text-rb-body-m`) as font-size utilities so they do not conflict with
+ * text-color classes such as `text-[var(--rb-text)]`.
+ *
  * @param inputs - Class values to combine (strings, objects, arrays, etc.)
  * @returns Merged class string with conflicts resolved
  *