v1.2.0 — completeness: Room cache, search, design pass

Completeness (android).

• Room offline catalog cache — books cached locally and served instantly, refreshed from GET /books (ADR 0002).
• ?q= text search wired into the catalog; the catalog-detail Borrow button is now wired.
• Design pass: docs/designs/*.html authored for every screen in the catalog design system and the Compose screens conformed. (On-device pixel verification is a manual Studio step — the headless CI host can't render a Compose emulator.)