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/components/Card.astro

@@ -1,17 +1,16 @@
 ---
 import { slugifyStr } from "@/utils/slugify";
 import type { CollectionEntry } from "astro:content";
+import { getPath } from "@/utils/getPath";
 import Datetime from "./Datetime.astro";
 
-export interface Props {
+export interface Props extends CollectionEntry<"blog"> {
   variant?: "h2" | "h3";
-  href?: string;
-  frontmatter: CollectionEntry<"blog">["data"];
 }
 
-const { href, frontmatter, variant = "h2" } = Astro.props;
+const { variant = "h2", data, id, filePath } = Astro.props;
 
-const { title, pubDatetime, modDatetime, description } = frontmatter;
+const { title, pubDatetime, modDatetime, description } = data;
 
 const headerProps = {
   style: { viewTransitionName: slugifyStr(title) },
@@ -21,7 +20,7 @@ const headerProps = {
 
 <li class="my-6">
   <a
-    href={href}
+    href={getPath(id, filePath)}
     class="inline-block text-lg font-medium text-accent decoration-dashed underline-offset-4 focus-visible:no-underline focus-visible:underline-offset-0"
   >
     {