feat(web): add nuxt-seo suite and page SEO metadata (NdoleStudio#941)

* feat(web): add nuxt-seo suite and page SEO metadata

Install @nuxtjs/seo (sitemap, robots, schema.org, og-image, link
checker) and add copywritten titles, descriptions and OG/Twitter tags
to all public pages via useSeoMeta.

- Disallow and exclude authenticated app routes from robots + sitemap
- Disable og-image (static ssr:false SPA has no Satori runtime)
- Approve vue-demi build script in pnpm-workspace.yaml

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix(web): add noindex route rule for /threads index

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore(web): remove redundant _robots.txt

nuxt-robots already generates Allow and Sitemap directives.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>