refactor: extract shared base for year/month/month-year dropdowns

[balajis-qb]

Summary

Structural cleanup — no linked issue. month_dropdown.tsx, year_dropdown.tsx, and month_year_dropdown.tsx were nearly identical and had already drifted apart in a way that broke accessibility for one of them.

Problem

• month_dropdown.tsx, year_dropdown.tsx, and month_year_dropdown.tsx were ~90% duplicated: same dropdownVisible state shape, same read-view / select-mode / scroll-mode / toggle skeleton — only the rendered options differed.
• That duplication had already let one copy silently drift and cause a real bug: month_year_dropdown.tsx's read view was a plain <div onClick> instead of a <button type="button"> like the other two, so it wasn't reachable via Tab and didn't respond to Enter/Space (tracked upstream as issue showMonthYearDropdown read view not reachable via Tab / keyboard Hacker0x01/react-datepicker#6311).
• Fix: extract the shared pieces (a DropdownReadView button, the BEM container class builder, and the scroll-mode array assembly) into src/dropdown_common.tsx, and have all three components use it. Unifying onto one read-view implementation fixes the div-vs-button bug as a structural side effect of removing the duplication, not as a separate targeted patch.

Changes

• Add src/dropdown_common.tsx: DropdownReadView (shared toggle button), dropdownContainerClassName, renderScrollModeChildren.
• src/year_dropdown.tsx, src/month_dropdown.tsx, src/month_year_dropdown.tsx: replace hand-copied read-view/container-class/scroll-mode JSX with the shared helpers.
• src/test/dropdown_common.test.tsx (new): direct test coverage for the extracted module.
• src/test/month_year_dropdown_test.test.tsx: add a regression test asserting the read view renders as a real <button type="button">.

Screenshots

Not applicable — This is a structural refactor with no visual change. The same CSS classes and markup shape are reused; the only element-tag change (div → button) is covered by tests rather than being visually apparent.

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.