refactor(nav): split Summit and Foundation headers into sub-components [INTORG-404]

[JonathanMatthey]

Summary

• Split SummitHeader.astro (379 lines) and FoundationHeader.astro (333 lines) into focused sub-components with co-located styles
• Extract shared navigation behavior (mobile toggle, submenu toggle, keyboard/click-outside handling) into src/scripts/header-nav.ts
• Move shared HamburgerButton to src/components/shared/ and shared MenuItem/MenuGroup types to src/types/navigation.ts
• Each header is now a slim orchestrator (~60 lines) composing NavMenu, NavActions, HamburgerButton, and a site-specific script

Related Issue

INTORG-404

Manual Test

• Verify Summit header navigation works (desktop + mobile toggle, submenus, active link highlighting)
• Verify Foundation header navigation works (desktop + mobile toggle, submenus, developer link highlighting)
• Test keyboard navigation (Escape closes submenus)
• Test responsive breakpoint at 1060px transitions cleanly

Checks

• pnpm run format
• pnpm run lint

PR Checklist

• PR title follows Conventional Commits (e.g. feat: ..., fix: ...)
• Linked issue included
• Scope is focused (target ~10-20 files when possible)
• Screenshots for UI changes (if applicable)

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[JoblersTune]

When this is in you should be un-blocked here #154

[Anca2022]

I made some changes to keep the translation work already done.

I have my doubts whether it's necessary to pass LanguageSwitcher classes as props

Some things are still broken:

• the active highlighting doesn't work (it only works for tech blog, and even there it's incomplete - it doesn't highlight the nav parent item when on a particular tech blog article)
• keyboard navigation doesn't work either - items hidden from view are reachable by tabbing, pressing ESC or clicking outside the open hamburger menu doesn't close it

But I think those should be handled as part of a different PR
I opened a new issue for those: https://linear.app/interledger/issue/INTORG-629/fix-navigation-on-v5-link-highlighting-and-keyboard-navigation

[Anca2022]

Suggested change

	Main Navigation
	{t('nav.main')}

Let's keep translations in place :)