Add keyboard navigation to date picker

[harsha573252]

Introduces a dedicated DatePickerKeyboardService to handle open/close, arrow/page/home/end navigation, mode-aware grid movement (day/month/year), disabled-date skipping, and focus restoration on Escape. Updates the picker templates/components for roving tabindex and active-cell focusing, converts the calendar icon into an accessible toggle button with aria-expanded state, emits mode changes to keep focus in the active grid, and fixes year-grid range start calculation using floor division. Adds broad unit/spec coverage for the new keyboard and focus behavior.

Frontend Pull Request Description

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• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have updated change log
• I have been following Definition of done
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new lint warnings
• New and existing unit tests pass locally and on CI with my changes
• Any dependent changes have been merged and published in downstream modules

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