feat(header): add Header component with mobile menu modes

[ndlabdev]

Summary

Adds the Header component: a responsive, sticky app header sized by h-(--ui-header-height), completing the height contract with Main and Error, with a mobile menu that composes the existing Modal, Slideover, and Drawer overlays.

Closes #148

Stacked on #150 (Main); merge that first, this diff then shrinks to the Header commit only.

Type of change

• 🐛 Bug fix
• ✨ New feature / component
• 📖 Documentation
• ♻️ Refactor / chore
• ⚠️ Breaking change

Changes

• Header component (src/lib/components/Header/): sticky translucent blurred <header> with title/to brand link, left/children(center, lg+)/right areas, top/bottom rows, all conditionally rendered
• Mobile menu via mode: 'modal' | 'slideover' | 'drawer' composing the existing overlays (focus trap, scroll lock, dismissal inherited); typed menu prop (HeaderMenuProps: side, direction, overlay, dismissible); body/content snippets forwarded
• Toggle button: toggle: boolean | ButtonProps, toggleSide, menu/close icon swap, aria-expanded, user onclick composed with the toggle behavior (preventDefault cancels opening), auto-hidden when there is nothing to open
• Bindable open; autoClose closes the menu on route change
• Hardening review applied with probe-first regressions: composed onclick, aria-expanded, no empty center area, menu option forwarding
• 37 browser tests; docs route /header; menu icon added to icons config; public exports (Header, HeaderProps, HeaderMode, HeaderMenuProps); CHANGELOG entry

Checklist

• Linked the related issue (Closes #…)
• pnpm check passes (0 errors, 0 warnings)
• pnpm lint passes
• pnpm test passes
• Added or updated tests for the change
• Updated CHANGELOG.md under [Unreleased]
• Followed component conventions (no comments outside *.types.ts, Material 3 design tokens)

Screenshots / notes

• The docs layout header is intentionally not replaced by this component yet (its hamburger opens the docs sidebar, a different pattern); planned together with a future NavigationMenu.
• Closes #148 does not auto-close on merge to dev; close manually after merging per repo convention.