fix(search): the live-search tooltip says what a search costs

It always said a live search spends the shared API quota, while the default source (the website's
own endpoint, scrape) spends none — the owner avoided a free search because of it. /api/me now names
the running search_source, and the tooltip names the cost under it: none for scrape (the enrich step
costs a unit or two), 100 units a page for the official API.

frontend/src/components/Header.tsx

@@ -60,7 +60,9 @@ const Header = forwardRef<
         }}
         onGo={() => onYtSearch(trimmed)}
         goLabel={t("header.searchYoutube")}
-        goTitle={t("header.searchYoutubeTip")}
+        goTitle={t(
+          me.search_source === "api" ? "header.searchYoutubeTipApi" : "header.searchYoutubeTip",
+        )}
         goIcon={<YouTubeIcon className="w-4 h-4" />}
         goDisabled={!trimmed || me.is_demo}
       />