[OF#4917] Update buttons styling (buttons position)

[vaszig]

Closes open-formulieren/open-forms#4917

The following screenshots show the new position of the buttons (mobile version already handles the correct order):

• Start page

• Form step

• Summary

[codecov]

Bundle Report

Changes will decrease total bundle size by 100 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
@open-formulieren/sdk-OpenForms-umd	4.75MB	50 bytes (-0.0%) ⬇️
@open-formulieren/sdk-esm	4.75MB	50 bytes (-0.0%) ⬇️

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.32%. Comparing base (49bd7cb) to head (9f211a5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #787   +/-   ##
=======================================
  Coverage   83.32%   83.32%           
=======================================
  Files         239      239           
  Lines        4750     4750           
  Branches     1268     1268           
=======================================
  Hits         3958     3958           
  Misses        757      757           
  Partials       35       35           

Flag	Coverage Δ
storybook	75.77% <100.00%> (ø)
vitest	62.64% <ø> (ø)

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