fix: clear price to null when a comparison finds no merchant offer

Several memory sticks (e.g. Crucial CT8G48C40U5, Patriot Signature Line,
Kingston FURY Beast) kept displaying their pc-builder.io catalog price
("€35.99", "€39.99"...) even after clicking "Compare prices" and getting
"No live price found, using catalog price" - an unconfirmed number was
being shown as if it were still valid, which is misleading given
pc-builder.io's catalog is known to go stale (see the earlier RTX 5070
€128 vs €66.89 case).

New clearPriceIfNoOffers(): when ScrapedPartsController#compare finds
zero offers across all 7 supported merchants, the part's price/priceSource
are reset to null, regardless of whether the price previously came from
the pc-builder.io catalog or an earlier verified merchant match - per
explicit product decision, an empty comparison always wins over keeping a
possibly-stale number around.

- ScrapedPartsController#compare now calls clearPriceIfNoOffers() in the
  empty-offers branch, and returns the resulting price in the JSON
  response so the frontend can mirror it exactly
- NewPartSearch.vue: comparePrices() now sets part.price from the
  response's `price` field (mirroring exactly what the backend just
  persisted) instead of only updating it on a cheaper find; removed the
  now-inaccurate "using catalog price €X" message since that price no
  longer exists once compared with no result
- 3 new tests for clearPriceIfNoOffers: clears an unverified
  pc-builder.io price, also clears a previously-verified merchant price
  when no longer confirmed, and is a no-op when there was no price to
  begin with

Verified end-to-end against the reported Crucial CT8G48C40U5 example:
price went from 35.99 (pc-builder.io) to null after a real comparison
found zero matching offers on any merchant.

inertia/pages/widgets/NewPartSearch.vue

@@ -62,21 +62,20 @@ async function comparePrices(part: ScrapedPart) {
     }
     const data = await response.json()
     offers[part.id] = data.offers ?? []
+
+    // The backend already persists this exact value server-side (see
+    // ScrapedPartsController#compare): the cheapest offer's price when
+    // one was found (updatePriceIfCheaper), or null when no merchant
+    // confirmed a price at all (clearPriceIfNoOffers) — an unverified
+    // pc-builder.io catalog price is not kept around just because the
+    // comparison came back empty. Mirror that here on the locally held
+    // result so the display (e.g. "€979.95" or "No price yet") reflects
+    // it immediately, instead of the initial search-result price sticking
+    // around until the user re-runs the search from scratch.
+    part.price = data.price ?? null
+
     if (offers[part.id].length === 0) {
       compareErrors[part.id] = true
-      return
-    }
-
-    // The backend already persists the cheapest offer found as this
-    // part's new best-known price when it beats the catalog price (see
-    // ScrapedPartsController#compare / updatePriceIfCheaper). Mirror that
-    // here on the locally held result so the displayed catalog price
-    // (e.g. "€979.95") reflects it immediately, instead of staying stuck
-    // at whatever price the initial search returned until the user
-    // re-runs the search from scratch.
-    const cheapest = offers[part.id][0]
-    if (part.price === null || cheapest.price < part.price) {
-      part.price = cheapest.price
     }
   } catch {
     compareErrors[part.id] = true
@@ -185,10 +184,7 @@ function importPart(part: ScrapedPart) {
             </a>
           </template>
 
-          <span v-if="compareErrors[part.id] && part.price !== null" class="text-xs text-gray-400">
-            No live price found, using catalog price €{{ part.price.toFixed(2) }}
-          </span>
-          <span v-else-if="compareErrors[part.id]" class="text-xs text-gray-400">
+          <span v-if="compareErrors[part.id]" class="text-xs text-gray-400">
             No price found on any merchant for this part.
           </span>
         </div>