add FTS5 full-text product search

Adds SQLite FTS5 search index with BM25 ranking across product title,
category, variant attributes, and description. Search modal now has
live results with thumbnails, prices, and click-to-navigate. Index
rebuilds automatically after each provider sync.

Also fixes Access syntax on Product/ProductImage structs (Map.get
instead of bracket notation) which was causing crashes when real
products were loaded from the database.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme/sync/product_sync_worker.ex

@@ -99,6 +99,10 @@ defmodule SimpleshopTheme.Sync.ProductSyncWorker do
       Products.update_sync_status(conn, "completed", DateTime.utc_now())
       product_count = Products.count_products_for_connection(conn.id)
       broadcast_sync(conn.id, {:sync_status, "completed", product_count})
+
+      # Rebuild search index after successful sync
+      SimpleshopTheme.Search.rebuild_index()
+
       :ok
     else
       {:error, reason} = error ->