feat(blog): Phase 5 SEO/GEO content with 5 new blog posts

Add 5 long-form Thai blog posts (1,200-2,500 words each) with SEO + GEO
optimization for the dealplustech water-systems site. Each post targets
a specific audience (contractors, engineers, project managers) and
follows a content-quality workflow: source real product specs, verify
Thai text, dedupe images, link back to product pages.

## New blog posts (src/content/blog/)
- thermobreak-guide.md (Thermobreak closed-cell insulation overview)
- plastic-grilles-guide.md (ABS plastic grilles for HVAC)
- ppr-pipe-guide.md (PPR pipe properties + heat-fusion welding)
- ppr-vs-hdpe-vs-upvc.md (3-way pipe comparison with PE80/PE100)
- thermobreak-series-guide.md (Thermobreak LS vs Solar series)
- 10-things-checklist-pipe-ordering.md (10-point pre-order checklist)

## Removed legacy posts
- pipe-knowledge.md, valve-guide.md, welcome-post.md (orphans)

## Hero images (public/images/blog/)
~20 product photos sourced from manufacturers (Thermobreak, Thai PPR,
thaiconsupply) plus Nano Banana Pro infographics. All resized to
3:2 aspect ratio per user preference. Source folder preserved for
re-derivation.

## Astro layout/SEO work
- src/components/seo/SEO.astro, JsonLd.astro (new SEO components)
- src/layouts/BaseLayout.astro, Layout.astro (OG/Twitter/JSON-LD wiring)
- src/pages/404.astro
- Product pages (8): added #pricelist anchors + schema work
- src/styles/global.css: scroll-padding for sticky-header anchors

## Automation scripts (scripts/)
- build_og_image.py (OG image builder)
- inject_faq_schema.py, inject_product_schema.py (JSON-LD injection)

## Misc
- public/robots.txt, public/images/og/default-og.jpg
- .gitignore: exclude scripts/__pycache__/

src/pages/404.astro

@@ -0,0 +1,84 @@
+---
+import BaseLayout from '@/layouts/BaseLayout.astro';
+
+// 404 — noindex, friendly UI with category shortcuts.
+const popularLinks = [
+  { name: 'ท่อ PPR ไทยพีพีอาร์', slug: '/ท่อ-ppr-thai-ppr' },
+  { name: 'ท่อ HDPE', slug: '/ท่อ-hdpe' },
+  { name: 'เครื่องเชื่อม HDPE', slug: '/เครื่องเชื่อม-hdpe' },
+  { name: 'วาล์ว', slug: '/วาล์ว-valve' },
+  { name: 'ปั๊มน้ำ', slug: '/water-pump' },
+  { name: 'กริลแอร์', slug: '/grilles' },
+  { name: 'ตู้ดับเพลิง', slug: '/ตู้ดับเพลิง' },
+  { name: 'Armaflex', slug: '/armflex' },
+];
+---
+
+<BaseLayout
+  title="404 - ไม่พบหน้าที่ค้นหา"
+  description="ขออภัย ไม่พบหน้าที่คุณกำลังมองหา กรุณาตรวจสอบ URL หรือเลือกเมนูที่ต้องการจากด้านล่าง"
+  robots="noindex, nofollow"
+>
+  <main class="bg-white min-h-screen flex items-center justify-center px-4 py-16">
+    <div class="max-w-2xl w-full text-center">
+      <!-- 404 graphic -->
+      <div class="relative mb-8">
+        <h1 class="text-9xl lg:text-[12rem] font-bold text-primary-600 leading-none">404</h1>
+        <div class="absolute inset-0 flex items-center justify-center pointer-events-none">
+          <div class="w-64 h-64 bg-primary-200 rounded-full blur-3xl opacity-30"></div>
+        </div>
+      </div>
+
+      <h2 class="text-2xl lg:text-3xl font-bold text-slate-900 mb-4">ไม่พบหน้าที่ค้นหา</h2>
+      <p class="text-lg text-slate-600 mb-8">
+        ขออภัย หน้าที่คุณกำลังมองหาอาจถูกย้าย ลบ หรือ URL ผิดพลาด
+      </p>
+
+      <!-- Primary CTAs -->
+      <div class="flex flex-wrap justify-center gap-4 mb-12">
+        <a
+          href="/"
+          class="inline-flex items-center gap-2 bg-accent-500 hover:bg-accent-600 text-white py-3 px-6 rounded-xl font-semibold transition-all hover:shadow-xl"
+        >
+          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M3 12l2-2m0 0l7-7 7 7M5 10v10a1 1 0 001 1h3m10-11l2 2m-2-2v10a1 1 0 01-1 1h-3m-6 0a1 1 0 001-1v-4a1 1 0 011-1h2a1 1 0 011 1v4a1 1 0 001 1m-6 0h6" />
+          </svg>
+          กลับหน้าแรก
+        </a>
+        <a
+          href="/all-products"
+          class="inline-flex items-center gap-2 bg-white border-2 border-primary-600 text-primary-700 hover:bg-primary-50 py-3 px-6 rounded-xl font-semibold transition-all"
+        >
+          <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 6h16M4 10h16M4 14h16M4 18h16" />
+          </svg>
+          ดูสินค้าทั้งหมด
+        </a>
+      </div>
+
+      <!-- Popular links -->
+      <div class="bg-neutral-50 rounded-2xl p-8 text-left">
+        <h3 class="text-sm font-semibold text-slate-500 uppercase tracking-wider mb-4 text-center">
+          หรือเลือกหมวดหมู่ยอดนิยม
+        </h3>
+        <div class="grid grid-cols-2 md:grid-cols-4 gap-3">
+          {popularLinks.map((link) => (
+            <a
+              href={link.slug}
+              class="block bg-white px-4 py-3 rounded-lg text-sm text-slate-700 hover:text-primary-700 hover:shadow-md transition-all text-center"
+            >
+              {link.name}
+            </a>
+          ))}
+        </div>
+      </div>
+
+      <!-- Contact hint -->
+      <p class="mt-8 text-sm text-slate-500">
+        ต้องการความช่วยเหลือ?
+        <a href="/contact-us" class="text-primary-600 hover:underline font-medium">ติดต่อเรา</a>
+        หรือโทร 090-555-1415
+      </p>
+    </div>
+  </main>
+</BaseLayout>