fix(navbar): preserve locale behind base path

[morluto]

Problem

getUserLanguage() reads the first URL segment as the locale. On GitHub Pages, the first segment is the deployment base path (qwen-code-docs), so navbar links can fall back to the browser language instead of preserving the locale in the current route.

For example, a visitor on /qwen-code-docs/fr/... can receive English navbar destinations when their browser language is English.

Changes

• strip the configured base path before reading the route locale
• keep browser-language fallback for routes without a locale
• add focused coverage for production, local, and locale-free paths

Validation

• cd website && npm test
• cd website && npx tsc --noEmit --incremental false
• autoreview against origin/main: clean

Build note

npm run build reaches Next.js compilation but currently fails on public/fonts/JetBrainsMono/JetBrainsMono-Variable.ttf with Unknown font format. That asset failure is present outside this navbar change and is not included here.