Add instant-search overlay (products + blog posts)

Separate commit on purpose so this can be reverted independently if
needed. Search icon in the Navbar (hidden below sm:, same reasoning
as WishlistLink — Account+Cart are the only always-visible icons on
true mobile) opens a debounced (250ms) overlay searching both
collections at once via a new /api/search route.

Plain Payload `where[...][contains]` queries (Postgres ILIKE), not a
real search index (Meilisearch/Algolia) — matches the catalog's
current small size, see [[project-ecommerce-sota-gaps]]'s own "search
becomes necessary past ~20 products" note. Worth upgrading later
without touching the overlay component itself.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/components/Navbar.tsx

@@ -7,6 +7,7 @@ import { usePathname } from "next/navigation";
 import { AnimatePresence, motion } from "motion/react";
 import { useCartCount } from "../lib/cart";
 import { useWishlist } from "../lib/useWishlist";
+import { SearchButton } from "./SearchOverlay";
 import { AUTH_CHANGED_EVENT } from "../lib/auth";
 import { NewsletterModal } from "./NewsletterModal";
 import { useCartFly } from "./CartFly";
@@ -544,6 +545,7 @@ export function Navbar({ singleActiveProduct, wishlistEnabled }: { singleActiveP
                 built-in padding read as too much space on mobile, where
                 these two icons are the only always-visible controls. */}
             <div className="flex items-center">
+              <SearchButton />
               <AccountLink />
               {wishlistEnabled && <WishlistLink />}
               <CartLink />