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/assets/icons/IconCalendar.svg

@@ -0,0 +1 @@
+<svg  xmlns="http://www.w3.org/2000/svg"  width="24"  height="24"  viewBox="0 0 24 24"  fill="none"  stroke="currentColor"  stroke-width="2"  stroke-linecap="round"  stroke-linejoin="round"  class="icon icon-tabler icons-tabler-outline icon-tabler-calendar-week"><path stroke="none" d="M0 0h24v24H0z" fill="none"/><path d="M4 7a2 2 0 0 1 2 -2h12a2 2 0 0 1 2 2v12a2 2 0 0 1 -2 2h-12a2 2 0 0 1 -2 -2v-12z" /><path d="M16 3v4" /><path d="M8 3v4" /><path d="M4 11h16" /><path d="M7 14h.013" /><path d="M10.01 14h.005" /><path d="M13.01 14h.005" /><path d="M16.015 14h.005" /><path d="M13.015 17h.005" /><path d="M7.01 17h.005" /><path d="M10.01 17h.005" /></svg>