The archives link in the site nav carries a text label

## Summary

Label the archives link in the header the way Posts, Tags, and About are labelled.

## Motivation

`src/components/Header.astro` renders Posts, Tags, and About as text links, then renders archives as an icon-only `LinkButton` — the label is `sm:sr-only`, so on desktop the only affordance is an unnamed archive glyph. The page that holds the entire post history is the least identifiable item in the nav, while `/posts`, which shows four at a time, gets the word.

[Rands](https://randsinrepose.com/) goes the other way and advertises it: "912 posts since 2002" in the footer, linking to the archive.

## Scope

- Give the archives link a visible label at desktop widths.
- Decide whether it keeps the icon alongside the label, and whether the mobile menu changes at all.
- Consider a footer pointer to `/archives` carrying the post count.

## Acceptance criteria

- [ ] The archives link is identifiable without hovering, at both mobile and desktop widths.
- [ ] `pnpm build` and `astro check` pass.

## Additional context

- #580 makes `/archives` the page that holds the whole history, which is what makes this worth fixing.
- #360 will churn `Header.astro` — worth sequencing against.