feat(a11y): Add skip-to-main-content link for keyboard accessibility (WCAG 2.4.1)

## Problem
OWASP Nest currently lacks a skip navigation mechanism. Keyboard and screen reader users are forced to navigate through repetitive navigation elements on every page load before accessing main content.

This violates WCAG 2.1 Success Criterion 2.4.1 (Bypass Blocks - Level A), which is a critical accessibility requirement.

## Solution
Introduce a visually-hidden "Skip to main content" link that becomes visible on keyboard focus. This allows users to bypass repetitive navigation and jump directly to the main content area.

## Why this matters
- Identified as a critical issue by axe-core ("bypass" category)
- Direct WCAG Level A violation (highest priority)
- Improves accessibility for keyboard and assistive technology users
- Aligns with OWASP Nest’s UI/UX Revamp (GSoC 2026 goals)

## Impact
- Fixes WCAG 2.4.1 compliance
- Improves Lighthouse accessibility score
- Enhances usability for all users, especially power and keyboard users

## Proposed Changes
- Add reusable `SkipLink` component
- Add `id="main-content"` and `tabIndex={-1}` to ``
- Ensure focus visibility and styling