Fix: Keep time panel aligned when todayButton is used with showTimeSelect

[balajis-qb]

Description

Linked issue: #6237

Problem
When both showTimeSelect and todayButton were used together, the time panel broke out of the normal layout. The today-button was rendered before the time section in the DOM, so the right-floated time panel got pushed below it instead of sitting next to the month view. This was "fixed" with a CSS hack that forced the time panel to position: absolute with a negative right offset — which detached it from the popper's layout box entirely. As a result:

• The time panel appeared visually detached/shifted, sitting lower than the date panel.
• Near the right edge of the viewport, the popper would shift to stay on-screen, but the absolutely-positioned time panel didn't shift with it and could get clipped or disappear.

Changes

• Reordered the DOM so the time section renders before the today-button (calendar.tsx), so both the month view and time panel stay as normal floats instead of needing the absolute-position hack.
• Removed the --with-today-button absolute-positioning hack and its related nav-arrow CSS exceptions in datepicker.scss, now that the time panel never leaves normal flow.
• The today-button now clears only the month view's column (clear: left + a --with-time-select width modifier) instead of spanning full width, so it sits directly under the calendar rather than under the time panel too.
• The time list's height now stretches to cover the month view plus the today-button's height (previously it only matched the month view), via the existing ResizeObserver-driven height sync in time.tsx.
• Extracted the repeated 85px time-container width into a $datepicker__time-container-width Sass variable instead of duplicating the magic number in four places.
• Added/updated tests covering the DOM order, the new CSS modifier class, and the height calculation that now includes the today-button.

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (548a1f3) to head (014f05f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6307      +/-   ##
==========================================
+ Coverage   99.29%   99.32%   +0.02%     
==========================================
  Files          30       30              
  Lines        3822     3824       +2     
  Branches     1648     1664      +16     
==========================================
+ Hits         3795     3798       +3     
+ Misses         26       25       -1     
  Partials        1        1              

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