docs: add route-level prefetching guide with GoRouter and AutoRoute patterns (#25)

[donfreddy]

Closes #25

Description

New recipe guide covering route-level prefetching patterns to eliminate loading spinners during navigation.

Content

• GoRouter redirect-based prefetching: fire-and-forget prefetch in route redirect callbacks, with error handling
• AutoRoute resolver-based prefetching: RouteGuard pattern with optional timeout grace period
• Cache-first vs network-first: strategy comparison and implementation patterns
• QoraPrefetch widget: in-tree prefetch for list items before navigation
• Testing section: deterministic prefetch tests with QoraClient directly
• Available in both EN (en/04.recipes/12.prefetch.md) and FR (fr/04.recipes/12.prefetch.md)