Add Easter egg overlay with falling eggs animation

[Null000]

Motivation

• Add a seasonal, time-limited visual effect to increase user delight around Easter 2026 by rendering falling egg emojis over the page.

Description

• Add CSS rules for a fixed overlay and falling items: .easter-overlay, .easter-rain, and @keyframes eggFall to control positioning, animation, and drop shadow.
• Insert an overlay container <div id="easter-overlay" class="easter-overlay" aria-hidden="true"></div> into index.html body.
• Add inline script that computes a date window around Easter 2026 (EASTER_2026) and, when in range, generates span elements with emoji content, randomized left, animationDuration, animationDelay, fontSize, and opacity to create the falling-egg effect.

Testing

• No automated tests were run or added for this change.

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Codex Task