SEARCH-001: Site-wide product search with keyword filtering and ranking

## Overview
Add a search bar to the site header. Users can search the product catalog by keyword. Results are ranked by relevance (name match > description match > category match).

## Acceptance Criteria
- [ ] Search input in the header (or a dedicated search page at `/search?q=`)
- [ ] Results update as user types (debounced, min 2 characters)
- [ ] No results state with helpful message
- [ ] Result items link to the product detail page (PDP-001)
- [ ] Search query reflected in page title and URL for shareability

## Depends On
- #36 (PDP-001 — Product Detail Page: result items link here)

Start by inspecting the site header, the product catalog data or entry point, and the product detail page referenced by PDP-001. Trace how product links and page titles or URLs are currently handled before choosing between header search and /search?q=. Done means debounced searches from two characters rank matches correctly, show a no-results state, link to product detail pages, and preserve the query in the title and URL.

Written by the indexing model from the issue text.