Pagination: page-size selector should compose Dropdown (follow-up to #30)

[lifeiscontent]

#30's Pagination uses a native <select> for the page-size selector as an interim. Replace it with the propel Dropdown (#16, merged) for visual consistency, same composition principle as the Table's editable cells. Ref: #30.

[lifeiscontent]

Done in #30 (commit 6b68506): the page-size selector now composes the propel Dropdown instead of a native <select>. The layer-3 pill is a DropdownTrigger labeled "N per page"; the menu lists the page sizes as single-select DropdownItems (current one checked), and picking one reports through the existing pageSize.onChange. The pageSize?: { value, options, onChange } API is unchanged; keyboard (ArrowDown/Enter opens, arrows + Enter select) and the labeled trigger keep it accessible, matching Figma 4762-503. Added a play-tested PageSizeSelector story. Closing in favor of #30.