Fix crash & safety issues in [slug] route

[tmshv]

Problems

1. Null pointer crash in generateMetadata — src/app/[slug]/page.tsx:17-18
page!.title and page!.excerpt crash the entire build if getPageBySlug returns null (malformed or missing MDX file). Add a null check and return fallback metadata.

2. CSS injection via themeColor — src/app/[slug]/layout.tsx:25-29
   Frontmatter themeColor is interpolated directly into a <style> block. Validate against /^#[0-9a-fA-F]{3,8}$/ before rendering.

3. getPageBySlug called 4 times per page — src/app/[slug]/page.tsx + layout.tsx
   Same MDX file is read and parsed in generateMetadata, Page, generateViewport, and SlugLayout. Wrap with React cache():

   import { cache } from 'react'
   export const getPageBySlugCached = cache(getPageBySlug)

4. PageDefinition has [name: string]: any — src/types.ts:15
   Index signature defeats TypeScript safety. Remove it and explicitly type all used frontmatter fields.