📝 docs(readme): lead with AI features + quire-definition tagline

[vitofico]

Bumps the README to advertise the AI feature set loudly instead of burying it, and adds a tagline rooted in the definition of quire.

Changes

• Definitional tagline under the title — quire (n.) — a gathering of folded leaves, bound into a book — tying the etymology to what Quire unifies: library + reading state + insights. The old privacy one-liner is preserved beneath it.
• New ## AI, on your terms section pulled up to right after "What it is", replacing the buried, flat "AI features (optional)" list. Leads with the two most concrete wins:
  • Book scan — barcode/ISBN → deterministic reader-affinity score (no LLM), owned-book detection, cover + optional full insight.
  • Grounded insights — LLM cards cited from Wikipedia/OpenLibrary, catalog-preview-before-download, shared cache.
  • Plus tone/language personalization, reader profile + 3 recommendation flavors, library stats, and self-hoster controls (budgets, health endpoint, deploy modes).
• Every AI claim is framed as off by default, per-user opt-in, routed through an endpoint you control — the privacy positioning and AI story coexist.
• Added an "Optional, private AI" bullet + jump-link to the stack overview in "What it is".

Docs-only; no code or CI changes.