fix(pipe-coupling): use MJG80A screenshot as hero image

User provided screenshot of RNT Smart Clamp MJG80A (with SUS304
material, NBR seal, 89.1mm pipe OD, 67.8-91.9mm range) as the
canonical hero image for this product.

Changes:
- public/images/pipe-coupling/mjg80a-hero.png: new hero image
  (copied from user's Desktop screenshot)
- pipe-coupling.astro: hero <img src> updated to mjg80a-hero.png
- all-products.astro: card image updated to mjg80a-hero.png

Other 5 model images (mjg/mjh/mjd/mjer/mjcx/rnt-e009) unchanged
as those are individual model photos from ranotech.com.

src/pages/pipe-coupling.astro

@@ -47,7 +47,7 @@ import BaseLayout from '@/layouts/BaseLayout.astro';
           </div>
           <div class="lg:sticky lg:top-24">
             <div class="rounded-2xl overflow-hidden bg-white/10 p-2 max-w-md mx-auto lg:max-w-none">
-              <img src="/images/pipe-coupling/smc-02.jpg" alt="RNT Smart Clamp — แคลมป์รัดท่อ Rano Tech" class="w-full max-w-md mx-auto lg:max-w-none" loading="eager" />
+              <img src="/images/pipe-coupling/mjg80a-hero.png" alt="RNT Smart Clamp MJG80A (SUS304 + NBR) — แคลมป์รัดท่อ Rano Tech" — แคลมป์รัดท่อ Rano Tech" class="w-full max-w-md mx-auto lg:max-w-none" loading="eager" />
             </div>
           </div>
         </div>