refactor: move blog from EmDash to Astro content collections

EmDash CMS integration is being removed. The blog content is
moved to native Astro content collections (markdown files in
src/content/blog/) which works with the static output config.

Changes:
- Remove EmDash from astro.config.mjs (revert to static output)
- Remove emdash packages from package.json/package-lock.json
- Remove seed/seed.json (was EmDash-only)
- Remove src/live.config.ts (EmDash Astro loader)
- Add src/content.config.ts (Astro content collection for blog)
- Move 3 blog posts to src/content/blog/*.md
- Update src/pages/index.astro to use getCollection('blog')
- Update src/pages/บทความ/[slug].astro to use render() from astro:content
  (Astro 6 API: render(article), not article.render())
- Update src/pages/บทความ/index.astro (blog list)
- Add .hermes/ to .gitignore

Verified:
- npm run build: 35 pages, complete in 2.50s
- / , /aeroflex, /about-us, /บทความ, /บทความ/welcome-post: all 200

src/content.config.ts

@@ -0,0 +1,15 @@
+import { defineCollection, z } from 'astro:content';
+import { glob } from 'astro/loaders';
+
+const blog = defineCollection({
+  loader: glob({ pattern: '**/*.md', base: './src/content/blog' }),
+  schema: z.object({
+    title: z.string(),
+    excerpt: z.string(),
+    featured_image: z.string().optional(),
+    published_at: z.coerce.date(),
+    tags: z.array(z.string()).optional(),
+  }),
+});
+
+export const collections = { blog };