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

package.json

@@ -18,6 +18,7 @@
     "@resvg/resvg-js": "^2.6.2",
     "@tailwindcss/vite": "^4.0.14",
     "astro": "^5.5.2",
+    "dayjs": "^1.11.13",
     "lodash.kebabcase": "^4.1.1",
     "remark-collapse": "^0.1.2",
     "remark-toc": "^9.0.0",