Make SubNavigation locale-aware for prefixed routes

[martimpassos]

SubNavigation built its sidebar tree by rooting at the first path segment of the current page (segments[0]). For locale-prefixed content (e.g. content/fr/...) that first segment is the locale, not the section, so the sidebar for a page like /fr/a-propos/guide was rooted at /fr and listed every page under that locale (including unrelated sections like /fr/chronologie) instead of just the current section.

• Added isLocaleSegment / rootDepthForSegments / rootSegmentForSegments helpers (using the existing canonicalizeLocaleCode) that root at <locale>/<section> (depth 2) when the first segment is a configured locale, else <section> (depth 1).
• Registered locale-prefixed <locale>/<section> nodes as navigation roots — previously only depth-1 nodes could ever become roots.
• Updated buildNavigationForFile and getPageInfo to use the locale-aware root segment.
• EXCLUDED_ROOTS (works/search) are now matched against the section name, so they stay excluded under a locale prefix (fr/works) too.