feat: announce page navigation - #51

Summary

• Use the localized main heading for the document title and announce page changes in a polite live region.
• Move focus to the destination heading once it is available and keep the initial render silent.
• Cover the initial render, navigation, delayed headings and language changes in tests.

Verification

• GitHub CI passed lint, formatting, typecheck, the full test suite and the production build.
• The local Ares accessibility suite passed all 110 tests with --testTimeout=30000. The default five-second timeout also reproduces on a clean checkout of main.

Fixes #21.