What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Closes #2902

Overview of changes:
On viewports below 768px, the top navbar now uses a hamburger menu instead of a horizontal scroll strip.

• Added a collapse toggler and vertical nav panel
• Kept the top row compact: logo, search, dark mode toggle, menu button on one line
• Fixed menu padding and link alignment (including slotted nav items via :deep())
• Fixed search / toggle / hamburger vertical alignment (flex layout + hidden search placeholder)
• Updated mobile dropdown styles so submenus expand inline
• Menu closes after navigation and when resizing back to desktop

Anything you'd like to highlight/discuss:

Testing instructions:

1. Build web assets: npm run build:web
2. Build and serve docs: cd docs && markbind serve (or build _site and serve locally)
3. At ~375px width, check:

• Collapsed header is one row; hamburger opens a vertical menu with padded links
• Search, dark mode, and hamburger are aligned
• Tapping a nav link closes the menu
• Dropdown items expand inline in the menu

4. At ≥768px, confirm navbar still looks and works as before
5. cd packages/vue-components && npm test -- --testPathPattern=Navbar

Proposed commit message: (wrap lines at 72 characters)

Replace mobile navbar scroll strip with collapsible menu
On viewports below 768px, hide primary nav links behind a hamburger
toggler with a compact single-row header. Fix mobile menu padding,
dropdown layout, and alignment of search and header controls.

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• NA

Give a brief explanation note about:

• what was the old feature that was made obsolete
  On screens narrower than 768px, the top navbar showed nav links in a horizontal scroll strip (second row, hidden scrollbar). Logo and search sat in a cramped two-row header.
• any replacement feature (if any), and
  Nav links are now behind a hamburger menu. The top row stays compact (logo, search, dark mode, menu button), and opening the menu shows links in a vertical list with normal padding.
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).