feat(ui5-calendar): improve header and day picker accessibility and focus behavior

[GDamyanov]

• Calendar header navigation buttons are now focusable via keyboard (tabindex set).
• Each header button has a descriptive title attribute for improved accessibility and user guidance.

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-12712--ui5-webcomponents-preview.netlify.app

[unazko]

Overall the code is clean.
I've found the following behavioural issue:

• Right mouse click triggers the previous/next button's action

[unazko]

The code looks good in my perspective.

There is one last issue:

• In the year range view the next/previous button tooltips should refer to year ranges instead of months.