feat: allow custom themes via registry extension

[rohilsurana]

Problem

`src/themes/registry.ts` hardcodes two themes (default, paper). There's no way for site owners to register custom themes without modifying Chronicle's source code. The theme type system (`ThemeLayoutProps`, `ThemePageProps`) is well-defined but not exposed for external use.

Suggested Approach

• Allow `chronicle.yaml` to reference a custom theme module path:
  ```yaml
  theme:
  name: custom
  module: ./themes/my-theme
  ```
• The module would export `{ Layout, Page, Skeleton }` conforming to the existing theme types
• Keep default and paper as built-in themes that don't need a module path