feat: scaffold Astro 7 with Tailwind 4, MDX, sitemap, node adapter and tooling

- Tailwind v4 via @tailwindcss/vite + typography plugin
- astro-icon (Tabler), self-hosted Rubik/Inter, RSS, Pagefind postbuild
- Prettier (astro + tailwind plugins), editorconfig
- new:post script with Turkish-aware slugify

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+indent_size = 2
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false