Fix Deepfield image preview with accessible modal

[karedas]

Changes

• Replace direct PNG links with an in-page native dialog, styled consistently with the portfolio.
• Add original-size zoom and fit-to-screen, close button, Escape and backdrop dismissal.
• Keep keyboard focus inside the dialog, restore it on close and lock background scrolling.
• Localize controls in Italian and English; keep screenshots visible without JavaScript.

Validation

• Astro check and build passed; Biome passed.
• 14 unit tests and 59 browser tests passed.
• Added desktop/mobile checks in both languages for zoom, keyboard interaction, accessibility, no downloads and no new tabs.

No CV or editorial content changes. Merge is left to the owner.

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