feat: allow blog posts to be organized by subdirectories

Blog posts can now be structured into subdirectories, which determine their slugs.

Example:
- `/src/data/blog/2025/testing.md` → `/posts/2025/testing`

Directories prefixed with `_` will be excluded from the slug.

Example:
- `/src/data/blog/_examples/tailwind-typography.md` → `/posts/tailwind-typography`

Closes #470, #366

src/layouts/PostDetails.astro

@@ -8,6 +8,7 @@ import Datetime from "@/components/Datetime.astro";
 import EditPost from "@/components/EditPost.astro";
 import ShareLinks from "@/components/ShareLinks.astro";
 import BackButton from "@/components/BackButton.astro";
+import { getPath } from "@/utils/getPath";
 import { slugifyStr } from "@/utils/slugify";
 import IconChevronLeft from "@/assets/icons/IconChevronLeft.svg";
 import IconChevronRight from "@/assets/icons/IconChevronRight.svg";
@@ -45,7 +46,7 @@ if (typeof initOgImage === "string") {
 
 // Use dynamic OG image if enabled and no remote|local ogImage
 if (!ogImageUrl && SITE.dynamicOgImage) {
-  ogImageUrl = `/posts/${slugifyStr(title)}/index.png`;
+  ogImageUrl = `${getPath(post.id, post.filePath)}/index.png`;
 }
 
 // Resolve OG image URL (or fallback to SITE.ogImage / default `og.png`)