feat: add support for Astro base path

[sirlisko]

Description

Adds support for Astro's base configuration option, allowing AstroPaper to be served under a sub-path (e.g. domain.com/blog).

A new SITE.base field in src/config.ts is the single place to configure this. It is passed to Astro's base option in astro.config.ts, which exposes import.meta.env.BASE_URL throughout the codebase.

All internal links in components, layouts, and pages now use BASE_URL instead of hardcoded paths.

Types of changes

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Documentation Update (if none of the other choices apply)
• Others (any other types not listed above)

Checklist

• I have read the [Contributing Guide](https://github.com/satnaing/astro-p
  aper/blob/main/.github/CONTRIBUTING.md)
• I have added the necessary documentation (if appropriate)
• Breaking Change (fix or feature that would cause existing functionality
  to not work as expected)

Further comments

To use a sub-path, set SITE.base in src/config.ts:

export const SITE = {
  base: "/blog/", // site will be served at domain.com/blog
  ...
}

Leave it unset or as "/" for a root deployment, behaviour is unchanged from before.

Related Issue

Closes: #493