Fix calendar/date picker UX

[sergei-maertens]

Partly closes open-formulieren/open-forms-sdk#863

Discussed with Laurens for the desired interaction:

• don't open the date picker on focus of the reference input (because the keyboard navigation inside the picker is bad at the moment, in the legacy renderer this is skipped over)
• the calendar icon to toggle the calendar supports click, enter and spacebar to open the calendar and can be reached with keyboard navigation

[codecov]

Codecov Report

❌ Patch coverage is 47.27273% with 145 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.37%. Comparing base (7f5b549) to head (7cfcc16).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/forms/DatePicker.tsx	37.75%	61 Missing ⚠️
...c/components/forms/DateTimeField/DateTimeField.tsx	28.33%	43 Missing ⚠️
...mponents/forms/DateField/DatePicker/DatePicker.tsx	26.66%	33 Missing ⚠️
src/components/icons/FontAwesome.tsx	87.17%	5 Missing ⚠️
src/components/icons/Icon.tsx	90.90%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   69.06%   69.37%   +0.30%     
==========================================
  Files         290      290              
  Lines       10769    10738      -31     
  Branches     1787     1778       -9     
==========================================
+ Hits         7438     7449      +11     
+ Misses       3327     3285      -42     
  Partials        4        4              

Flag	Coverage Δ
storybook	89.32% <84.26%> (-0.03%)	⬇️
vitest	54.33% <27.27%> (+0.34%)	⬆️

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[sergei-maertens]

Had a chat with Laurens - the desired behaviour is to only open the picker on click of the icon rather than on focus of the reference text input. Then we can make the button role="button" again too.