feat: public marketing site (vitrine)

Adds a (marketing) route group -- Home, Features, About, Privacy,
Terms, Contact -- reusing the app's design system/i18n/deploy
pipeline rather than a separate site. Root page.tsx now branches on
session instead of always redirecting to /recipes: logged-in visitors
still land in the app unchanged, logged-out visitors see the vitrine
home page.

The actual integration point: proxy.ts's PUBLIC_PATHS previously sent
every anonymous "/" request straight to /login before page.tsx's
redirect ever ran. Added the new marketing routes to PUBLIC_PATHS,
plus a separate exact-match list for "/" itself -- it can't go in the
startsWith-matched array, since every path starts with "/" and that
would make the whole app public.

Also adds app/sitemap.ts and app/robots.ts (neither existed before),
disallowing the authenticated app and the unlisted /r/ and /s/ share
routes from crawling while allowing /u/ public profiles.

Pricing page deliberately not included -- waiting on Stripe Checkout
(STRIPE_PLAN.md) so its CTA goes somewhere real. Privacy/Terms are
structural drafts flagged inline as not lawyer-reviewed, per
STRIPE_PLAN.md's own tax-advice caveat -- same spirit here.

Verified with a full production build (pnpm build) -- compiles clean,
all 7 new routes render, since there's no DB in this sandbox to run
the dev server against for a real click-through.

v0.48.0

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.47.0",
+  "version": "0.48.0",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",