feat: add หัวจ่ายแอร์ Ball Jet product page

New product page at /หัวจ่าย-ball-jet covering Ball Spout Jet
Diffuser (JD Series) — long-throw ceiling air diffuser for
high-ceiling commercial/industrial buildings.

Content sourced from sapamaket.com, sapaengineer.com (official),
superaircool.com technical specs, and the user's brief. The page
covers:
- Hero with full feature badges (360° rotation, 45° nozzle, sizes,
  materials)
- Product description, features, technical specs, applications,
  advantages
- Image gallery (7 photos scraped from the official SAPA source at
  sapaengineer.com — public product marketing images, used to
  illustrate the product)
- 5-question FAQ
- Contact CTA

Nav updated: added หัวจ่ายแอร์ Ball Jet as the 3rd subcategory under
'อุปกรณ์ปรับอากาศ' in BaseLayout.astro.

No PDF pricelist (none provided by source). No price data displayed
(per user request — pricing to be confirmed separately).

src/layouts/BaseLayout.astro

@@ -49,6 +49,7 @@ const categories = [
     subcategories: [
       { name: 'กริลแอร์', slug: '/grilles' },
       { name: 'DURGO วาล์วเติมอากาศ', slug: '/durgo-avvs' },
+      { name: 'หัวจ่ายแอร์ Ball Jet', slug: '/หัวจ่าย-ball-jet' },
     ]
   },
   {