fix: SEO requires description but IndexTemplate omits it

[jimmyandrade]

Summary

Opening the home page logs:

Warning: Failed prop type: The prop `description` is marked as required in `SEO`, but its value is `undefined`.
    at SEO (…/metax/dist/…)
    at SEO (src/components/SEO/index.js)
    at IndexTemplate (src/templates/index.js)

Root cause

metax SEO marks description as required. IndexTemplate (and some other templates) pass title / titleTemplate but not description. Site metadata already has a default description in config/siteMetadata.

Acceptance criteria

• Local SEO wrapper defaults description (and title if omitted) from useSiteMetadata()
• Home page / IndexTemplate no longer emits the PropTypes warning (explicit description + wrapper default)
• Unit tests cover the wrapper defaults
• Upstream metax PR: fix(metax): fall back SEO description to siteMetadata muy/muy#155 (muy/muy#154)

Resolution

Shipped in #456 (consumer). Upstream open: muy/muy#155.

[jimmyandrade]

Consumer fix in #456. Upstream metax: muy/muy#154 / muy/muy#155