a11y: add modal semantics and focus trap to EasterEggTerminal

[cevheri]

Problem

EasterEggTerminal.astro overlay behaves like a modal but is missing:

• role="dialog" + aria-modal="true" on the container
• Focus management (move focus into dialog on open, restore on close)
• Focus trapping (prevent tab into background content)

Fix

Add proper ARIA attributes and focus management to the terminal overlay.

Priority

Low — the feature is intentionally hidden (Easter egg).

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Source: PR #11 Copilot review (round 2)