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/config.ts

@@ -17,4 +17,6 @@ export const SITE = {
     url: "https://github.com/satnaing/astro-paper/edit/main/",
   },
   dynamicOgImage: true,
+  lang: "en", // html lang code. Set this empty and default will be "en"
+  timezone: "Asia/Bangkok", // Default global timezone (IANA format) https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
 } as const;