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/content.config.ts

@@ -19,6 +19,7 @@ const blog = defineCollection({
       description: z.string(),
       canonicalURL: z.string().optional(),
       hideEditPost: z.boolean().optional(),
+      timezone: z.string().optional(),
     }),
 });