Improve mobile navigation accessibility

[Plattnericus]

Summary

• Adds expanded/hidden state attributes to the mobile navigation trigger and overlay.
• Treats the mobile menu as a labelled modal navigation dialog.
• Locks body scrolling while the menu is open, moves focus to the close button, restores focus to the trigger when closed, and supports closing with Escape.
• Adds Playwright coverage for the mobile menu open and close behavior.

Validation

• Ran ./node_modules/.bin/prettier.cmd --check src/components/Navigation/Navigation.astro src/components/Navigation/navigation.ts tests/navigation.spec.ts.
• Ran git diff --check.
• Transformed src/components/Navigation/navigation.ts and tests/navigation.spec.ts with esbuild to verify TypeScript syntax.

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