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

database/migrations/1784808177630_alter_components_table.ts

@@ -0,0 +1,17 @@
+import { BaseSchema } from '@adonisjs/lucid/schema'
+
+export default class extends BaseSchema {
+  protected tableName = 'components'
+
+  async up() {
+    this.schema.alterTable(this.tableName, (table) => {
+      table.string('purchase_url').nullable()
+    })
+  }
+
+  async down() {
+    this.schema.alterTable(this.tableName, (table) => {
+      table.dropColumn('purchase_url')
+    })
+  }
+}