🎨 Palette: Enhance Calendar Header Accessibility

[endlessblink]

💡 What: Added :aria-expanded to the view options dropdown trigger, applied role="group" and an aria-label to the view selector (Day/Week/Month) container, and added :aria-pressed dynamic bindings to the view selection buttons.
🎯 Why: The calendar view selector functions as a custom tablist or segmented control, but previously lacked the semantics for screen readers to announce the grouping, the purpose of the control, or the currently active view. The dropdown trigger also lacked state announcement.
📸 Before/After: Visuals remained identical. See verification screenshot for UI stability.
♿ Accessibility: Improved screen reader navigation and comprehension for core calendar views.

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PR created automatically by Jules for task 3527028715958441232 started by @endlessblink

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