🎨 Palette: Add keyboard spacebar and semantic roles to anchor buttons

[0m364]

💡 What: Updated #sitrep-btn anchor tags across all HTML files to include role="button" and aria-haspopup="dialog". Also bound a keydown event in modal.js to ensure the Spacebar triggers the modal properly while preventing default scrolling behavior.
🎯 Why: Native HTML anchor tags (<a href="#">) behave like links instead of buttons to screen readers, and do not natively respond to the Spacebar key for interaction. This ensures keyboard and screen reader users can predictably trigger the modal.
📸 Before/After: Visuals remain unchanged. A Playwright test in test_ui.py has been added to prove keyboard accessibility.
♿ Accessibility: Ensures semantic correctness (role="button") and standard keyboard interactivity (Spacebar support) for links used exclusively as UI triggers.

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PR created automatically by Jules for task 5742326161916662310 started by @0m364

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