feat: track purchase site URL for new components

- New nullable purchase_url column on components
- Store/update validators accept an optional purchaseUrl (must be a valid
  absolute URL with a protocol when provided)
- components/create.vue: NewPartSearch import now also fills purchaseUrl
  from the scraped part's source URL (pc-builder.io product page); the
  field remains manually editable for second-hand parts entered by hand
- components/edit.vue: purchaseUrl is editable after creation
- components/show.vue: displays the purchase site as a clickable link
  when present
- 4 new functional tests: create with/without a purchase URL, invalid URL
  rejected (asserts the Inertia inputErrorsBag flash, not a raw 422 -
  content negotiation only returns JSON for XHR/API-style requests), and
  updating an existing component's purchase URL

app/models/component.ts

@@ -24,6 +24,9 @@ export default class Component extends BaseModel {
   @column()
   declare computerId: number | null
 
+  @column()
+  declare purchaseUrl: string | null
+
   @column({ serializeAs: null })
   declare userId: number