feat: add global and per-post timezone support (#491)

* refactor: replace embedded svg with svg icon

* feat: add global and per-post timezone support

- add `SITE.timezone` in config to define a global default timezone.
- allow per-post timezone overrides via `timezone` in frontmatter.
- update date formatting logic to use the correct timezone when rendering timestamps.
- ensure that posts without a timezone fallback to `SITE.timezone`.
- improve consistency between local and deployed timezones.

* refactor: replace `LOCALE` constant with  `SITE.lang`

* docs: update guides

* fix: update code word-break inside markdown table

Closes #466

src/layouts/PostDetails.astro

@@ -29,6 +29,7 @@ const {
   canonicalURL,
   pubDatetime,
   modDatetime,
+  timezone,
   tags,
   hideEditPost,
 } = post.data;
@@ -97,12 +98,7 @@ const nextPost =
       {title}
     </h1>
     <div class="flex items-center gap-4">
-      <Datetime
-        pubDatetime={pubDatetime}
-        modDatetime={modDatetime}
-        size="lg"
-        class="my-2"
-      />
+      <Datetime {pubDatetime} {modDatetime} {timezone} size="lg" class="my-2" />
       <EditPost class="max-sm:hidden" {hideEditPost} {post} />
     </div>
     <article id="article" class="mx-auto prose mt-8 max-w-3xl">