docs: add Layout Blueprint, Recipes, and Deployment pages

Layout Blueprint: explains the .app-wrapper / .app-header / .app-sidebar
/ .app-main / .app-footer regions with an ASCII diagram, region-by-
region reference, body-level layout modifier table, responsive
behaviour notes, and common pitfalls. This is the page that was
missing for developers asking "how does the page structure actually
work?".

Recipes: copy-paste solutions for the patterns that appear in every
admin dashboard -- multi-level sidebar with badges + headers, active
state highlighting from JS, responsive tables, user dropdown,
small-box KPI cards, breadcrumbs, timelines, direct-chat widget,
chart-resize-on-sidebar-collapse hook, fetch-into-card, and a
confirm-destructive-action modal.

Deployment: production-ready guidance covering minified vs dev
builds, compression, cache headers, CDN integrity hashes, script
loading order (the failure mode for getting it wrong was previously
undocumented), defer vs async, critical CSS, preload hints,
service-worker offline support, and rsync excludes that strip
source maps and the Astro intermediate output.