YEC 2024 screenshots

Screenshots for strings added in !116 (merged). For tor-browser.ftl.

screenshots/yec-2024-main.png is for:

• yec-2024-introduction-0
• yec-2024-please-donate
• yec-2024-matched-donation
• yec-2024-donate-button

screenshots/yec-2024-close.png is for:

• yec-2024-close-button

screenshots/yec-2024-alt1.png is for:

• yec-2024-introduction-1

screenshots/yec-2024-alt2.png is for:

• yec-2024-introduction-2