refactor: upgrade to V25, replace overlay with popover

[web-padawan]

WIP, do not merge.

Depends on #12

This is a PR for the actual V25 upgrade. There are still some more things to do:

Done

• Replaced custom overlay implementation with vaadin-popover wrapping calendar
  • This provides out of the box styling and significantly simplifies component theming
• Moved button elements from vcf-month-picker-calendar shadow DOM to slots
  • Both use vaadin-button with theme="vcf-month-picker" for easier styling
  • Needed to ensure that global Aura CSS applies to these buttons if necessary
• Moved year label to the slot as well, to make aria-labelledby ID reference work
• Added part attributes to month grid and cells in vcf-month-picker-calendar

TODO

• Remove remaining Lumo styles from JS files (src / theme) and add a CSS file instead
• Add some base styles tweaks e.g to make toggle button look correctly with base styles