Add locale-aware config and localized framework UI defaults

[khashashin]

This PR adds project- and page-level locale support to Framework and localizes Framework-owned UI defaults.

Changes:

• add locale, lang, and dir to Framework config
• add front matter overrides for locale, lang, and dir
• set root HTML lang / dir during render
• localize Framework-owned default UI strings, including:
  • home
  • contents / table of contents label
  • untitled page fallback
  • search placeholder
  • sidebar toggle title
  • pager labels
  • default footer text and date formatting
• update docs and tests

This intentionally keeps translation catalogs and fallback policy out of Framework core; those remain user-land concerns.

This is the Framework-side companion to the Plot locale work, keeping translation catalogs and fallback logic out of Framework core.

[Fil]

Is it global, or is it possible to have a multilingual project? Where some articles would be in English and other in Arabic?

[khashashin]

@Fil yes, page-level overrides are supported, so you can have a multilingual project where different pages use different language/locale settings.

This is handled via front matter support here: https://github.com/observablehq/framework/pull/2054/changes#diff-77b3519370ff48f6caf7c316f0ece966f2f0871c40e9381d451d2c5b1c76b40fR113

For example:

---
lang: ar
dir: rtl
locale: ar-EG
---

So you can have one page in English and another in Arabic within the same project.

One current limitation is that some config-derived defaults remain project-level unless explicitly overridden, such as the default home label and default TOC label. If that should also become page-localized, that could be a follow-up.