Twenty Fourteen: Fix dropdown menu touch support and accessibility issues

[R1shabh-Gupta]

Trac ticket: https://core.trac.wordpress.org/ticket/30575

Note
This PR is a continuation of PR #8820 and addresses additional issues outlined in Trac ticket #30575, comment 13.

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Checklist

• Fix: Sub-menu cannot be closed by clicking/tapping outside (like Twenty Sixteen behavior).
• Fix: .focus class not applied when resizing screen after load below breakpoint (affects accessible focus styles).
• Fix: Make CSS (783px) and JavaScript (781px) breakpoints consistent.
• Add: Check if the menu exists before running the function to prevent issues in child themes or themes without menus.

[github-actions]

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