feat: allow custom remark/rehype plugins via config

[rohilsurana]

Problem

`src/server/vite-config.ts` hardcodes the remark/rehype plugin chain (directives, admonitions, image resolution, link resolution, mermaid, reading time, GFM, frontmatter). Site owners can't add custom plugins (e.g. remark-math, rehype-katex) without forking.

Suggested Approach

• Add an optional `plugins` field to `chronicle.yaml`:
  ```yaml
  plugins:
  remark:
  - remark-math
  rehype:
  - rehype-katex
  ```
• Resolve plugin modules from the project's `node_modules`
• Append user plugins after the built-in chain